Absolute minimum for a D&D 5E DM to know?

Question

Every year on January 1st, for over 15 years, my group of friends have been doing a "New Year's RPG session". I've played and DMmed my fair share of 3E, 3.5E, and D20 Modern, but I've stopped playing about 4 years ago. My friends have kept on playing and now prefer 5th edition.

This January 1st, I volunteered to DM a 5th edition "one shot". My question is simple (though possibly a tad bit broad):

What are the very very basic pieces of rules I should know about when DMming 5th edition?

To make it less broad, assume:

• Someone reasonably experienced at DMming (including "winging it" if needed)
• A fully fleshed out, custom, home-brew one-shot story ready to go (backgrounds, maps, etc all taken care of by me)
• Helpful, cooperative players, several of whom have DMmed 5th edition themselves
• Near-zero risk of power playing, rule abuse, and the like from players
• Level 5 characters
• Adventure with intended mix of combat, skills, puzzles, and role playing

It's hard to "know what you don't know", but judging from earlier editions, I'm particularly worried about things like these from 3e and 3.5e:

• Things like passive abilities (like ones to detect secret doors) I should know about
• Certain skills being "opposed", where the DM should roll for a player, hiding the result
• What reasonable DCs are (e.g. what a reasonable AC would be, or a reasonable break DC for a door)
• Combat rules that you'd really want to read up on, ahead of time (e.g. Grapple was so nontrivial, that if you have a grappling enemy in the story you'd better read up in 3.5e)

My players have all the books, and they've also provided me with the 5th ed. SRD (pdf), but I have neither time nor interest to read through it all (right now, at least).

Answer 1

For a DM well versed in previous editions, I would highlight the following:

Setting DC and AC

Bonuses to skill checks and AC do not increase much as characters level, thus AC/DC "grades" can be assumed to be constant.

low/medium/high AC: 12/16/19

basic/challenging/hard DC: 10/15/20

Any kind of target number over 20 will be quite a hurdle even for high level characters.

Passive rolls

There are only two skill you have to look out for here: perception and insight. The former taking the place of search and notice, the latter of sense motive. What previous editions called "take 10" 5e calls a passive check, with the value unchanged. Rolls made to hide or lie are usually made against the appropriate passive check. You should write down these values for PCs and NPCs alike.

New mechanics

Advantage

Situational modifiers have been practically compressed into the advantage/disadvantage mechanic. You roll the d20 twice and take the higher or lower result as appropriate. Note that this does not change the range of the possible results and thus supports the static AC/DC scales. Giving a +x based on circumstances is not advised.

Concentration

Stacking spells is also very limited and thus the bonuses gained are constrained. Most buff and debuff spells require concentration and a single character can only concentrate on one spell at a time. Concentration can be broken by dealing damage to the caster (DC10 Con save to keep it in nearly all cases if your PCs are level 5).

Combat

The rules are much more streamlined in 5e. You should read the list of conditions in the PHB, starting on p.290 and spanning a bare 3 pages with copious amounts of illustrations. Other minor things:

• There is no flanking. (It is an optional rule.)
• Only retreating from melee provokes an attack of opportunity. (Can be avoided by taking the Disengage action.)
• Movement on one's turn can be broken up. (Movement is not an action.)
• Turns cannot be delayed, you can only ready a single action or movement (PHB p.193).
• On your turn you can move and take 1 Action. Some creatures (mostly PCs) can use certain features in addition to this and that is called a bonus action.

Answer 2

You should read "Part 2: Playing the Game" in the rulebook

You may be looking for something shorter, but I recommend at least skimming through, and preferably reading through at least once, the entire "Part 2: Playing the Game" of the rulebook. It's in the free online Basic Rules, and consists of chapters on "Using Ability Scores", "Adventuring", and "Combat". If you have to leave one of the chapters out, you probably need "Adventuring" the least. The online rules have about the same text as the full player's handbook, but those 21 pages (pages 57–77 in the Basic Rules PDF) cover all the mechanics of play, and I think reading through all of it is the best way to prepare.

If the players are experienced, you don't really need to look at character creation or the details of spells, as they'll let you know if their character has a relevant ability. So, you just really need to understand ability checks (While the names of the six core attributes have stayed the same across D&D editions, their meanings have evolved substantially.), and how the mechanisms of combat work. The rules of play themselves are fairly concise.

And don't be afraid to toss rules questions back to the players, if they're more experienced with the system than you are. And as with anything, if your group isn't sure on a particular ruling, just make a decision and do what's more fun at the time and look it up later.

Answer 3

Know the rules you want to use

There is no clear "minimum" of rules that will definitely be used in a game. It's entirely possible that you could have an entire session, or even an entire campaign where no one rolled dice even once. Thus, the minimum rules needed depends mostly on the kind of story that you and your players intend to tell.

Assume that your players know their own characters: it is their responsibility to learn the rules concerning the mechanics of actions they will commonly do (how spell slots work for casters, how AC works for martial classes, etc.). So don't feel the need to read up on every level progression or special skill that your players might have.

Similarly, it is your responsibility to have read up on the rules on actions that you intend for the creatures in the world to frequently do. If you think NPCs will attack, learn the rules on attacking combat. If you think NPCs will take some dice rolls to be convinced, look up the rules on using skills.

If you're asking for a set of rules that we think will often come up, my complete shot in the dark advice would be:

• Read up about actions you can take on your turn. In particular, become comfortable with the distinction between Action, Bonus Action, Reaction, and Movement. Also learn how the READY action works, since it is somewhat counterintuitive.
• Learn the basics of combat. They will work mostly like you expect.
• Learn how spellcasting works. Particularly, look up CONCENTRATION, since it is a major mechanic in this edition.

Again, what you need depends largely on what you intend to do. The things I mentioned above are mostly to make sure that your rulings don't break what's called "action economy": this edition works hard to determine how many things each player can do in each turn, and balance that with how powerful each player's actions are. Try to familiarize yourself with the major points of that, but otherwise I'd just suggest you learn what you plan to use. After all, your players can help you with the rules on anything THEY plan to do.