Can I respond to someone ending their second main phase, or is that not really a thing? Like this situation:
Player 1: Let me know when you are ending your main phase (has mana floating)
Player 2: Ok, ending my main phase.
Player 1: *casts a spell with flash or and instant*
Player 2: I now cast *some non instant card or card without flash*.
Yes, that is legal.
In order to end a step or phase, all players must pass priority in succession without taking any actions while the stack is empty. When player 2 announces that they are ending the main phase, what they are actually doing is passing priority with the intent of moving on with the game, but player 1 receives priority when this happens. If player 1 takes some action at this time, then it is still the main phase and the active player will receive priority again after the spell player 1 cast resolves. They are under no obligation to pass priority again without doing anything simply because they did so before.
In short: The end of a phase isn't something you can respond to such that the phase then ends after your response has resolved. You are simply doing something with the priority you receive instead of passing and causing the phase to end.
You can't "respond" to ending the phase, however you (every player) must pass priority before the phase can end, meaning you have the opportunity to do things.
Basically, Alice has priority during her main phase. She wishes to end the phase and move to combat, so she passes priority to Nathan. Nathan uses some floating mana to cast a spell. Because Nathan didn't pass priority, priority returns to Alice and the phase doesn't end. Alice may do things with the priority (including cast sorcery-speed stuff, since we're talking about the main phase) before passing back to Nathan.
Once every player passes priority, the game moves to the next step or phase.
In a practical game, Alice would generally say something like "Moving to Combat", and Nathan would say "Before the end of your main phase, I..." If Nathan's actions change Alice's decision to move to combat right now, she can remain in the main phase.
Player 1: Let me know when you are ending your main phase (has mana floating)
Player 2: Ok, ending my main phase.
Player 1: *casts a spell with flash or and instant*
Player 2: I now cast *some non instant card or card without flash*.
Is this sequence of plays legal?
Mono-Black Oona, Queen of the Fae
Athreos, God of Passage
In order to end a step or phase, all players must pass priority in succession without taking any actions while the stack is empty. When player 2 announces that they are ending the main phase, what they are actually doing is passing priority with the intent of moving on with the game, but player 1 receives priority when this happens. If player 1 takes some action at this time, then it is still the main phase and the active player will receive priority again after the spell player 1 cast resolves. They are under no obligation to pass priority again without doing anything simply because they did so before.
In short: The end of a phase isn't something you can respond to such that the phase then ends after your response has resolved. You are simply doing something with the priority you receive instead of passing and causing the phase to end.
Basically, Alice has priority during her main phase. She wishes to end the phase and move to combat, so she passes priority to Nathan. Nathan uses some floating mana to cast a spell. Because Nathan didn't pass priority, priority returns to Alice and the phase doesn't end. Alice may do things with the priority (including cast sorcery-speed stuff, since we're talking about the main phase) before passing back to Nathan.
Once every player passes priority, the game moves to the next step or phase.
In a practical game, Alice would generally say something like "Moving to Combat", and Nathan would say "Before the end of your main phase, I..." If Nathan's actions change Alice's decision to move to combat right now, she can remain in the main phase.
Two Score, Minus Two or: A Stargate Tail
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