must not

• mustn't

must not (third-person singular simple present must not, no present participle, no simple past or past participle)

1. Used to indicate that something is forbidden.

   Synonyms: must refrain from, be not to

   Pedestrians must not cross the road until the light goes green.

Although must and have to are synonymous, there is a distinction in English between must not (indicating proscription) and not have to (indicating a lack of necessity).

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