Can I use "mustn't" to reply the question ? I know usually when we ask the question: May I smoke in this room ? ,the reply can be : No, you mustn't If I use "Must "to start a question ,for example : Must I get to school before 9:00am ? usually the negative answer is : no ,you needn't and we don't say " no, you mustn't " .This is the grammar we learn in China . But some Chinese students are arguing: if we are in some specific situation ,we can reply " no ,you mustn't " to the question " Must I..." Here is the imagined situation: Bob and Mary are a couple ,they were on Titanic ,Mary found the ship was about to sink, she can hardly hold the ship's rail cuz the ship was leaning , but Bob couldn't reach her , then Mary asked : Must I jump into the sea ? , How would Bob reply ? of course he didn't want Mary to jump into the sea,he wanted her to hang on in there ,can Bob say: NO, YOU MUSTN'T or he should say: NO,YOU CAN'T Thank you for your helpOr when Bob and Mary asked the third person: Must we jump into the sea ? can the 3rd person say: NO,YOU MUSTN'T ?