Set a Firm Rule: Disagreements about Rulings are to be handled between Sessions, not during Sessions

Generally speaking, a player should not interrupt the DM to quibble about how rules work. What the DM says is final, and if players don't agree with how the DM is handling rules, they should wait until after the session is over to resolve disagreements.

So with this player, you need to be very clear and direct in telling them that, even if they're totally right, they need to wait until the session is over to discuss mistakes or changes to the rules. Then, based on that discussion, you can agree to retcon certain events in the prior session, or you can agree to follow different rules going forwards, or you can assert that you're going to be following your understanding of the rules instead.

But make sure you understand the rules as well as possible

As DM, you have a responsibility to make sure you understand the rules as well as you can. Players have specific class features, actions they can take during combat, spells they can cast, and so on, and so forth. If you don't follow the rules to the best of your ability—or you don't explicitly signpost when you're deliberately not following the rules-as-written—it makes it harder for players to make informed decisions about how to play their characters.

This isn't even necessarily limited to changes to the rules or homebrew material: A Wizard will probably keep Identify in their spellbook to help identify magic items they acquire, and if you're using the variant rules for Magic Item Identification (DMG, pg. 136), where the Identify spell alone isn't enough to fully reveal the properties of a Magic Item, they might get upset if you don't inform them that you plan to use those rules ahead of time, since that was a character creation decision they made under false premises. Now, that's a relatively minor example, and easily handled at most tables ("Alright, I'll let you swap that spell out from your spellbook with a different 1st level Wizard Spell for free") but more substantial deviations could be more frustrating to players, because if the rules aren't well defined, then players are going to feel like they can't make meaningful decisions.

You should probably avoid homebrewing mechanics if you're new to the rules

Related to the above, introducing new mechanics without fully considering how they interact with the world they've been introduced to can have negative consequences.

One example that came up in a recent campaign I participated in was my DM's decision to add a new creature type to the game to represent the central villains of the campaign. I believe the DM's intentions were to create a villain that was suitably alien to the Faerun setting, and arguably, they succeeded. But the issue then became that this new creature type had no interaction with any of the abilities or features that any other character had:

• They were a NEWTYPE, so spells that say things like "affects Aberrations, Celestials, Fey, Fiends, Undead, and Elementals" wouldn't affect them.


• Their special abilities were of NEWTYPE, so, for example, a Paladin Aura that says "gain Resistance to Spell Damage" wouldn't affect their "Spell-like" abilities
  
  
  • This also meant their features bypassed things like Anti-Magic Fields
  
  


• Their Damage Type was NEWTYPE, meaning no feature that grants Damage Resistance (or Vulnerability!) could interact with it, unless it read something like "Resistance to ALL damage", which is a very small subset of features

As a result, it was impossible, as players, to make meaningful decisions with respect to how to prepare for fights with these kinds of creatures. We couldn't revise class features without just homebrewing them, and we couldn't prepare spells to deal with these creatures because most of our spells wouldn't work on them at all.

Eventually, we were able to convince the DM to incorporate NEWTYPE creatures as a presumed addition to the "Aberrations, Celestials, etc." lists that many spells have, which allowed us to actually make tactical decisions when approaching these kinds of encounters.

^{Also in general he's dramatically dialed back how frequently they show up in encounters; we've gone more than a dozen sessions since the last time we ever fought a creature of that type.}

The point being, while homebrew mechanics can make the game more interesting, or handle scenarios that the base rules don't handle especially well, you have to spend time making sure that they interact with the rules in a balanced and fun manner, or you'll end up creating mechanics that either dull down the game, or rob your players of meaningful agency.

Getting better at handling rules

One thing that I do as DM is I'll spend a lot of time trying to examine edge-case scenarios in D&D to better understand the interactions of the rules. Sometimes, if I see something strange, like a long-range archer improving their attack odds by blinding themselves, I'll expressly decide that not following the rules-as-written is necessary to improving the game.

But here's the trick: even if I have no plans to follow the rules-as-written in a specific scenario, it's still important that I understand what the rules are, and why they were written that way, because that helps me better understand what problem I'm trying to solve, and better justify my decision to not follow that rule.

So given that these disagreements over rules are affecting not just your relationship to this player, but your relationship to other players, it's important you make sure you understand what the rules are, and develop a good justification for not following those rules as written. Planning encounters when you don't know what kinds of mechanics might interact with them is going to end in frustration, even if the players know better than to express that frustration in real-time at the table.

Perhaps have a conversation with your players about what kind of game they want to be playing

D&D historically has its roots in Wargaming gameplay mechanics. So as a broad principle, a player "playing to win" is not a strictly invalid method of playing. D&D, even 5th Edition, facilitates this kind of metagamey, "here's how we strategize to win!" gameplay, and many groups that don't play in this manner often do so by eliding mechanics of the game that they feel don't contribute to the gameplay style they want.

However, the evidence strongly suggests that that's not the kind of game you want to be playing. So what's very important is that you sit down with your players and decide what kind of game you want to be playing. If all of you are in agreement that you want to be playing a Wargame type game, or if you're all in agreement that you want to be playing a more social, less combat-focused game, then things will be good. But if you're each trying to play different games, then you're only going to have further problems.

Finally: Communicate with players

A lot of issues at the table can be resolved by openly and honestly discussing what isn't working, and what needs to change.

I've emphasized as much through implication, but if you're willing to talk to your players about what frustrates you about the gameplay, and they're willing to listen, I'm confident you'll find a solution that works for everyone.

Communication is key

I'm definitely a player (and DM) who likes to try and go by the rules as much as possible. But not only are mistakes sometimes made, sometimes those mistakes aren't mistakes at all. Whatever the reason for the debate, here are the ways I've approached it:

As a player, when I think a mechanic was done incorrectly

My table generally does bring up potential mechanical issues as soon as they occur. Yes, this gives a quick pause and we may look up the rule to verify.

There are times when the DM has said "This is what's happening and we're moving forward." At that point, I've learned to just drop it. Yeah, I may sulk. But I'm going to move on during the session and I may (or may not) bring it up later after the session to get more clarification.

Other times, the DM has said "Whoops, you're right. It's X." This is fine, too. A mistake was made and it was corrected without having to retcon because we questioned it immediately.

But that doesn't mean that there may be an at-table mechanic that isn't RAW. Unless a session zero has already established zero deviation from RAW, then there is always going to be some give here. You'll have to trust your DM in that they're making their choices to tell a good story. If you really feel they aren't, then it's time to talk with them outside about what you feel.

As a DM, when I've made a mistake

I am by no means perfect. I make mistakes. I actually want to be told to hold up a second to do something right so that I can correct it. But there are also times where I've had to make a ruling.

For instance, in one game there was an insect swarm. The bard wanted to cast Vicious Mockery and I wasn't sure that insects could even hear. We talked about it as a table, and I let it happen. I was leaning the other way, but we did some quick research and I decided to roll with it. Had I gone the other way, the bard would have been unhappy, but I think they'd have moved on.

As a DM, when I haven't made a mistake

There have also been times where the player challlenged a mechanic and I was comfortable in my knowledge that I had done it right. In those cases, I've gently said "We can check the rules later, but this is what it is and let's move on."

The player may not like it, and they may sulk. But at least I acknowledged the concern but tried to keep the game going.

We're here to have fun

Ultimately, we're all doing this to have fun. We're not always right, but we are always certain at the time about how we feel. Letting people voice those feelings and giving them a response that you've heard them is the least that can be done and should be done. How to respond to each circumstance is going to depend on that circumstance. But just make sure to create an atmosphere where people are listened to and that everyone is enjoying the game.

Moving forward, talk with your players with a Session Zero reset if you haven't done one. Or do a quick recap with them about the rules, their application, and your freedom as DM and that there should be trust that everyone is trying to have fun.

I don't know whether you promote the players vs DM mentality, but if you do let them know that while that atmosphere is here, it's to be done in a playful way. Yes, there's a challenge, but that challenge is friendly.

There's a lot to unpack here. I guess I'll go in order.

Everyone's game styles are different (in at least some amount).

I prefer more of a "Follow the rules as they are written" style. I also know that not everyone feels that way.

When I'm a player, I tell the DM that I'm going to point out something in the rules and answer questions. If there's a change to the rules, that's fine, just let me know and I'll mentally pack it away.
For example:

Party member: How does healing work during short rests?
Me: You can roll as many hit dice as you have, one at a time.
DM: Goodguy ,actually, I feel that using all of your hit dice in a single rest is a bit too strong, we're only using up to half your hit dice per rest in this game.
Me: Okay. Got it. Sorry about the confusion.

No problemo. I've relayed the information that I know and then move on. This relationship has worked pretty successfully in at least 3 D&D 5e games. The only issues I've run into were before hashing this out with the DM in question.

When I'm a DM, I explain to the players (in session zero) that I try to follow the rules as a baseline and I might improvise special mechanics as needed. If I get a rule wrong in the middle of combat or something, note it and let me know about it later; we're not going to ret-con the past 5 minutes because you were able to read line 5 on page 196, or whatever. I have had zero problems with this method across about a dozen players.

It's never too late to explain your position and/or (re)do a session zero.

You may have to talk to this player aside

If everyone has talked and agreed on a playstyle moving forward... and the player continues to be a problem, you should have a conversation with them. It's important to use neutral language and avoid things like "you're ruining my game". You'll want to stick to things like "I'm hoping this can be a collaborative story telling experience. There's no winning or losing, we're all just trying to have an enjoyable evening/morning/extra-temporal experience."

(Disclaimer: I'm not the best talker, so those might not be the right words.)

Don't let one player ruin your hobby

I don't know if you're a good DM or a terrible DM. You might be awful, who knows? We were all bad DMs at one point or another.

The only two things you need to be able to be a great DM are:

• Enjoying the hobby


• A willingness to learn

That's it. If one likes being the Dungeon Master (you'll always be able to find players) and one doesn't think that they're too good to learn, then one can get better. Whether that means going from bad to mediocre, or from good to great, or anything in between.

The problem is not with you. Base on your description he has to understand that

The authority of the GM is above the rules

He is wielding the written rules against you like it is some higher authority you, as GM, have to answer to. You have to explain to him, in no uncertain terms that that is not the case. The final arbiter at the table is the GM.

I do not have enough information to say why he is doing this. My first guess would be that he is (maybe unconsciously) afraid that you will screw him over, that the rules are "hidden" in your head and can change without him knowing. This would not mean that he does not trust you as a person, just that he is not yet used to the social dynamic of a tabletop RPG.

He has to learn, and you have to make sure that he can trust the GM. Stick to the rules whenever possible. If you can not or do not want to, inform the players of the rules regarding the particular situation before they make their decision about it. It is important to be consistent about this. If he can expect this from you, his fear should be alleviated.

Of course, you should not make him just shut up. That would just make things worse. Also, you can make use of his expertise with the rules and he might feel appreciated if he can help. In particular, there are two things he should be allowed to do:

1. Offer helpful suggestions if you are stuck on how to rule.

Not having to look at the book can speed play up considerably. The second part is important. He has to stop initiating this. He is there to assist, not to decide.

2. Articulate his expectations based on the written rules.

If he has made a decision based on the written rules, this fact may surface only later, when it would bear fruit. If you just missed a rule, yield to him. If you have decided to rule against the rules, either explain your reasoning (briefly), or point it out that his character also finds the situation strange (if there is something hidden that you are taking into account).

It is possible that the source of this behavior is something else. But whatever the case, being consistently transparent about rulings as a GM is necessary. It may take some time for him to get used to it, but it will serve as the basis of GM-player trust. And will also benefit the other players, who might not be so vocal, as well as the overall atmosphere of your games.

Being consistent and reliable has allowed me to avoid this situation. I was given the task to basically introduce a group to D&D. During the first couple of sessions it was apparent that they were "testing the waters", so to speak. To see what they can get away with. Doing crazy things and asking whether the rules let them do it. Being transparent this way has allowed for this atmosphere to transform into one of trust between us, and I cannot imagine any of these players coming to hit me over the head with a rulebook like this.

Also, in most cases, I am the one at the table with the deepest knowledge of the rules. As a player, I do not argue about every little thing with the GM, because I know I can trust them and we can discuss things if they truly matter. I have to admit that faced with a GM ruling arbitrarily or springing "gotcha" moments on me, I can imagine myself getting the urge to cite the rules at them and "correct them", to get a grip on how the in-game world works.

The old D&D 4th edition Dungeon Master's Guide has good advice on this

Although you're playing D&D 5th edition, the earlier D&D 4th edition Dungeon Master's Guide's advice on problem players is still useful and relevant. Particular to your current predicament is the description of how to handle a rules lawyer (p.32):

You don't have to be a rules expert to be the DM, but that doesn't mean one other player should assume that role. A rules lawyer is a player who argues against the DM's decisions by referencing the rules. You should welcome players who know the rules. They help when you're stuck or you make mistakes. But even helpful rules lawyers become a problem if they correct you or continually give you rules advice that's just wrong. Much worse are players who can't stand negative results, and who comb the rules for loopholes and misinterpretations that their characters can exploit.

You mention that you learned the rules in an ad-hoc manner, rather than rigorously learning and applying the rules. This was widely considered a valid DMing approach in early editions of D&D, but players of modern editions (3e, 4e, 5e) increasingly expect to be able to leverage fixed and certain rules.

While knowing all the rules isn't mandatory to DM, you mention a feeling that your group disagrees with your approach. This may suggest that you need to adapt your DMing style to incorporate more rigorous rules knowledge in order to give your group the experience they want.

The 4e DMG continues:

A table rule about holding rules discussion until the end of game is enough to dissuade some rules lawyers. Stay open to minor corrections, though, as long as they're not too frequent.

This is generally a good approach to keep the game running swiftly. When a rules dispute arises, make a ruling on the spot, promise to look the correct rule up after the game, and continue play. You have to stick to this! It's not reasonable to stop the game for twenty minutes to debate a rule.

You should, of course, be open to minor rules corrections from players, particularly if they come from a player whose rules knowledge is often correct. Some DMs will nominate one trustworthy player as an impartial rules expert. It doesn't matter so much whether you invent a ruling or accept a player suggestion, as long as it's handled swiftly and decisively so that the game can continue.

If a player insists on stopping to look up a rule, the 4e DMG has this elegant solution:

If the game grinds to a halt while a rules lawyer tries to find a specific rule or reference, invite the player to take as long as he wants to search for it while you and the rest of the players continue the game. The rules lawyer's character essentially steps out of the game for as long as it takes. Monsters don't attack him, and he delays [takes no action] indefinitely. This solution makes the other players happy, because they get to keep playing D&D instead of letting one player stop the game.

With regards homebrew material, the player has to accept that it is not possible for them to know every possible monster statblock and rules element ahead of time. Inventing new game elements isn't cheating! You don't need to justify why this particular monster behaves differently to a similar creature in the Monster Manual.

A particular controversial phrase is "you can't do that". You can always do that. You're the Dungeon Master. The rules are your tools, but they do not bind you. The game runs by the laws that you set down, not the rules in the book. Your answer to "you can't do that" must always be "Yes, I can. I'm the Dungeon Master."

The player may follow up by saying, "But it's not in the rules." You can respond to this simply by saying, "No, it's not."

In short, you don't have to follow all the rules in the book, and you certainly don't have to let players control you by casting rules at you. However, it would be beneficial for you to learn and use the official rules, at least the ones that come up often, as many players consider it more fair to play by the book as much as possible.

As a DM and as a player, I am of the firm belief that the DM is god. They are above the rule book. However, as a DM, when I set up a game, I will clarify any rules which are in play or out of play in that campaign. Obviously, I can't go through the book page by page detailing my conditions, but giving players an idea of what types of rules from the book will simply be ignored and which will be more strictly adhered to helps set the tone for any discussion later.

The game is meant to be fun, so being open to a bit of flexibility is often the key. If a player asks "Can my character do this? I know you don't really like but I think it would be fun under these circumstances." Players giving me a good story for bending a rule or allowing something outside my preferences are usually given some leeway, though not always to the conclusion they hoped for (I need to be entertained too after all). Players who expect me to abide by their mental image of the rule book generally have a harder time adapting to my style.

26 years later since my first D&D game and I still can't but be surprised from seeing people being unable to distinguish between a game and a job. And do not take it personal, this is not about you, but more about a wide problem that you can find in 50% of the questions here on RPG.SE

Anyway, tl;dr:

You are both wrong.

So, you have decided that you like being a DM, ok, good. Thus you give to other people your availability to run a game. Good.

And because you are the one who runs the game you decide the setting, and then how you want to master, and then everything else. You pick which rules you like, and which not. What's allowed and what's not. And then you can even decide that you are available to master a game only on the odds days of the even months, and only if the moon is rising. It's your time, it's your game, it's your effort and, more than everything else, it's your offer.

It's not a job. It's not slavery. It's not mandatory.

Now, on the other side, a bunch of people want to play a game of D&D. Good. They decide what they want, they decide which style of playing they like, they express preferences, maybe they like a setting more than another, and then they check the different available offers in term of DMs in their area. They may be searching for a very strict master, for a storytelling oriented, for a fully homebrew system...doesn't matter. In the end, they have a demand.

And that's all. Demand and offer: I still have to see a DM or a player holding someone at gun point forcing them to be a master or to be a player.

So your question of what to do has a quite simple answer: you have decided how you want to run your game (it's your free time you are investing in it, after all, right?), if someone doesn't like it he has all the rights to drop out and find another master(it's their free time they are investing in it, after all, right?). Nobody is paying anyone, there is no contract of D&Ding, there are no masters or players Trade Unions, it's just a hobby.

Isn't life already enough complicated by itself, and already plenty of things we are forced to do? Why the need to complicate and pollute your free time too?

An aside, my personal opinion -which, by the way, doesn't conflicts with the above.

I don't know what is the issue here, but when I got my first D&D box it took me less than a week to learn all the rules by memory. The same with the other following boxes and then with AD&D manuals (which, I must admit, I never wanted to play and never did). I can see someone new to the game needing a couple of weeks to adjust to it, to read the books a couple of times, understand what the fuss is all about, and then clearly it takes a couple sessions or three of playing to get a decent grasp of it. Then:

1. You can't, two years later, still struggle with rules. You should have read the rulebooks a hundred of time, know nearly everything by memory, and be able to open them at exactly the page you need in any moment.

Okay I admit I'm new to D&D. I've been playing for 2 years now and
only been playing 5e

No, you're not new, after two years you are a senior on D&D. You're everything but new. Two weeks is new.

2. You write:

I don't learn by reading the entire manual in one sitting, I have
learnt everything from actually playing the game through trial and
error and observing others.

And this reads as

"I've taken a role of responsibility; I've decided to be the one who
take charge of running a game; I've stepped up to wear the mantle of
the one who must have complete knowledge of D&D because he's the sole
responsible of the good running of things...and I've decide that the
best way to do it is by skipping the part where I know what I'm doing.

And then, obviously:

I like to think it has worked, but I get the feeling my group
disagrees.

Why should they agree? You have offered them to play a game which is explicitly designed to have a strong master who leads and manage all the complexity and the rules, and you do it by willingly ignoring the rules and having any idea of what you're doing. What reaction did you expect?