The Order of Combat
Combat runs in rounds; each round is about 6 seconds of game time. The order is fixed:
- Determine surprise. The DM compares the stealth of ambushers against the passive Perception of everyone else. A surprised creature can't move or act on its first turn, and can't take a reaction until that turn ends.
- Establish positions. The DM decides where everyone is — this can be theater of the mind or a grid.
- Roll initiative. Everyone rolls a Dexterity check. The DM ranks combatants high to low — that's the initiative order, and it stays the same for the whole fight.
- Take turns. In initiative order, each combatant takes a turn. When everyone has acted, the round ends and a new round begins.
Your Turn
On your turn you can move up to your speed and take one action. You decide whether to move before or after your action, or split your movement around it. You can also:
- Take a bonus action — but only if a spell, class feature, or other ability says you can.
- Interact with one object for free (draw a sword, open a door). A second interaction costs your action.
- Do anything else the DM allows as part of movement and action — talking, shouting a warning, and similar flavor are free.
Movement
You can move a distance up to your speed, in any increments. Moving through a hostile creature's space is difficult — you generally can't move through an enemy's space unless the creature is two sizes larger or smaller than you, or it's incapacitated. Difficult terrain costs 2 feet of movement per foot. You can drop prone for free (standing costs half your speed).
Reactions
Certain abilities let you act outside your turn — the most common is the opportunity attack: when a hostile creature you can see moves out of your reach, you can use your reaction to make one melee attack against it. You get one reaction per round, which refreshes at the start of your next turn.