Combat
Combat runs in rounds — six seconds of story each — and on your turn you move and take one action. The full chapter is @Missing file; the action menu is @Missing file.
Initiative
When blades come out, everyone rolls a Dexterity check and acts in descending order. Surprise works the classic 2014 way: a surprised creature can't move or act on its first turn, and can't take a reaction until that turn ends. Ambushes are worth planning.
Your turn
Move up to your speed, act once. The core actions: Attack, Cast a Spell, Dash (move again), Disengage (leave reach safely), Dodge (attacks on you have disadvantage), Help (grant an ally advantage), Hide, Ready (hold your action for a trigger), Search, Use an Object. Some features grant a bonus action; you also get one reaction per round — most famously the opportunity attack against enemies who leave your reach without Disengaging.
Attacking
Roll d20 + ability modifier + Proficiency Bonus against the target's AC. Melee runs on Strength, ranged on Dexterity — finesse weapons let you choose. On a hit, roll the weapon's damage dice and add the same ability modifier. A natural 20 is a critical hit: roll all damage dice twice. Your sheet prints your exact attack bonus and damage for whatever you're holding; read them off.
Cover and conditions
Half cover grants +2 AC and Dexterity saves, three-quarters +5, total cover can't be targeted. The fifteen Conditions — Blinded through Unconscious — each have a file stating exactly what they change; when one lands on you, the GM applies it from the file, not from memory.
Dropping to zero
At 0 hit points you fall unconscious and start making death saving throws: d20, no modifiers — 10+ succeeds, three successes stabilize you, three failures kill you. A natural 20 brings you back on 1 hit point; a natural 1 counts as two failures, and damage taken while down adds failures too. Healing of any amount ends the ordeal instantly, which is why the cleric keeps @Missing file prepared and why you keep the cleric alive. The grim details: @Missing file.