Definition from Wiktionary, the free dictionary
Jump to: navigation, search

this entry is missing the "negation of must" like in "we must go, mustn't we?"—This comment was unsigned.

True, but the same use exists for must not: google books:"she|he|i|we must must we|he|i|she not". I'm not sure whether the definition of mustn't should be generalized from "must not; used to indicate that something is forbidden" to simply "must not", or whether on the other hand another sense should be added to [[mustn't]]. Either way, I think a usage note at [[must]] is in order.​—msh210 17:25, 18 February 2010 (UTC)
We don't need another definition. This is just a tag question. It's just a characteristic of modal verbs that they can be used thus: compare "you can't come, can you", "I should be there, shouldn't I/should I not?", and the list goes on --Rising Sun talk? 17:32, 18 February 2010 (UTC)
But this is different from those in that "shouldn't I" and "I shouldn't" are both negations of "I should" whereas "mustn't I" is a negation of "I must" but "I mustn't" is a negation of "I may".​—msh210 17:53, 18 February 2010 (UTC)
I agree with Rising Sun. This is simply a grammatical structure known as a tag question. It is used for all auxiliary verbs, whether modal or not, and has very little to do with the usual meaning of the negative form. BTW, "mustn't" is the negation of various possible clauses, and simply refers to refusal of permission. OTOH, I would recommend placing a see Appendix:English tag questions in all auxiliary verb affirmatives and negatives. I will set this appendix up now. -- ALGRIF talk 12:07, 20 February 2010 (UTC)