must 和 have to 在表示“义务”和“推论”时具有相同的意义。但是，它们的否定式是否也具有相同的意思呢？
作“必须”解的 must, 其否定形式 must not(mustn't), 并不是“不必要”的意思, 而是“不许”(表示绝对禁止)的意思:
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You mustn't park here. (You are not allowed to park here.) 你不许在这儿停车。
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You mustn't go swimming today. 今天不许你去游泳。
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You mustn't say things like that in front of your mother. 在你母亲面前你绝不可以说这样的话。
因此，当表示“不必要”的意思时，只能使用 don't have to 或 needn't。
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He doesn't have to go there. (Millington) 他不必去那里。
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They didn't have to worry about money. (Mansfield) 那时他们不必为金钱操心。
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You need not dust them. (Mansfield) 你不必拂拭它们。
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So you needn't hurry with your meeting. (Galsworthy) 你不必急着去参加会议。
must 作“一定,准是”解时,其否定式有时采用 must not(mustn't),虽然曾遭到批评,但被越多的人所接受,尤其是在美国英语中:
His absence must not have been noticed. (= His absence can't have been noticed.) 他的缺席一定没有被人注意到。
用于这个含义时,人们更喜欢使用 can't:
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That can't be the postman—it's only seven o'clock. (PEU) 这绝不会是邮递员——才7点钟。(比较:That must be the postman)
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You can't be telling lies. (CGEL) 你不可能在说谎。
(参见 mustn't/can't)