Turn Structure, Priority & the Stack

Every turn has five phases, in order:

1. Beginning — Untap step, Upkeep step, Draw step.
2. Precombat Main — the first main phase.
3. Combat — Beginning of Combat, Declare Attackers, Declare Blockers, Combat Damage, End of Combat.
4. Postcombat Main — the second main phase.
5. Ending — End step, then Cleanup.

Nothing happens during the Untap step and no player receives priority there. Sorcery-speed actions (sorceries, most creatures, "as a sorcery" abilities) can only be played during your own main phase while the stack is empty.

Priority is permission to take an action. Whenever priority is passed:

• The active player (whose turn it is) receives priority first in each step/phase — this is the APNAP order (Active Player, Non-Active Player).
• A player with priority may cast a spell, activate an ability, take a special action, or pass.
• If all players pass priority in succession with the stack empty, the current step or phase ends.
• If all players pass with something on the stack, the top object of the stack resolves, then the active player gets priority again.

The stack is the zone where spells and abilities wait to resolve. It works last-in, first-out (LIFO): the most recently added object resolves first. This is why a response can resolve before the thing it is responding to.

Only instants, spells/abilities with flash, activated abilities, and triggered abilities use the stack. Mana abilities and special actions (like playing a land, or turning a face-down creature face up) do not use the stack and cannot be responded to.

Before a player would receive priority, the game first checks state-based actions (e.g. a creature with 0 toughness is put into the graveyard) and then puts any waiting triggered abilities onto the stack. Only after that is priority actually given. This ordering matters constantly in real games.