mustn't

must +‎ -n't

• IPA^(key): /ˈmʌs.n̩t/

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mustn't

1. must not (negative auxiliary[1]); used to indicate that something is forbidden or, in a tag question, that something is not necessary.

   You mustn't blame yourself for this tragedy.

   We must go, mustn't we?

   • 1906, Edith Nesbit, The Railway Children, Chapter 4: The engine-burglar,

     "Now, listen," said Mother; "it's quite true that we're poor, but we have enough to live on. You mustn't go telling everyone about our affairs--it's not right. And you must never, never, never ask strangers to give you things. Now always remember that--won't you?"

   • 1989: Batman (movie)

     Look, we mustn't mistake ourselves for regular people. We're artists.

Although must and need are synonyms, mustn’t and needn’t (including their uncontracted forms) are not, except when used interrogatively. “You mustn’t go” means you must stay, but “you needn’t go” means you do not have to go, with the implication that you still may if you wish.

• shouldn't
• can't
• oughtn't

• Appendix:English modal verbs
• Appendix:English tag questions