magic what CAN you do on the opponent's turn

[Buster]

what CAN you do on the opponents turn in magic?

[Aspirations]

So long as your cards don't say otherwise...

You can play instants.

You can use activated abilities you control.

You can block the creatures he may attack you with.

You can pretend to ignore what he's doing in an effort to intimidate him. Then when he does something like attack, you can be like, "Oh, sorry. I was thinking about something else." This will also frustrate him into thinking you don't even care about this game and don't consider him a challenge. His anger will make him play more hastily and carelessly. He will be more apt to make mistakes. Muahahahaha...