這要根據(jù)情況而定.
例①it must be a girl
變否定：it 【can't】be a girl
在這里must表肯定推測,意思為‘一定’ 表示否定推測的詞為‘can't'
例②You must eat and drink in the classroom
變否定：You【 mustn't】 eat and drink in the classroom
此處句子并沒有判斷的意味,而是一種帶命令口吻的,此時(shí)變否定就應(yīng)該直接用原形+n't了,mustn't譯為：禁止、不可以.常指規(guī)章制度等.
例③You must rerurn the book to the library at once
變否定：You 【needn't】 rerurn the book to the library at once
可以與②進(jìn)行一下比較,②這件事是一件壞事,是說話人必須要絕對制止的,而③這件事性質(zhì)就不同了,是你不需要怎么怎么樣.