must变否定句,只需要在 must 后面直接加not ,变为 must not 缩写为mustn't；
变疑问句,直接将must提到句首.例如：
1、——Must I come over to help you with your work tomorrow ?【must 必须】
——Yes,you must/No,you needn't
2、——Can I take the book out of the reading room?
———No,you mustn't.【禁止,不许】
3、——There is someone coming in this way.Who can it be?
——It must be Tom【准是,一定】
—— It can't be him.He is seen on the way back from here just now.
【“不可能”要用can't或couldn't,不能用mustn't】
祝你开心如意!O(∩_∩)O~~
