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I prefer, "what you mustn't do".
Cecil, you mustn't do that to your sister!We mustn't let him find out or it'll ruin the surprise.
The contraction is mustn't (mŭs'ənt)
Contraction of must not.
The 't' (mustn't). You would say it as "musn't".
If you were to say it in full, "must not", you wouldpronounce the 't'.
The contraction mustn't means "must not."