1) need表示需要，主要用于否定句和疑問句。用于疑問句，肯定回答用must，否定回答用needn’t。have to表示他人或客觀情況要求"必須"，也可以翻譯為"不得不"，have to否定形式意思是"不一定"或"沒有必要"。
【例如】
you needn’t come if you don’t want to.
i don’t think you need be so polite to her.
you don’t have to work so hard if you just want to pass band 4.
because of the heavy rain, the children have had to get up earlier than usual this week.
it was very kind of you to do the washing-up, but you ____it. (cet-4 1998,6)
a) mustn’t have done
b) would not have done
c) might not have done
d) didn’t have to
本句譯為：你把這些東西洗了，你真是太好了。但是你沒有必要洗。must, would, might后面接動詞的完成式，都很少用否定式，而didn’t have to表示沒有必要，符合題意，故答案為d.
2) need后面接動詞的完成式，主要用于否定句，表示本可不必做某事。
【例如】
you ____ all those calculations! we have a computer to do that sort of thing. (cet-4 1991)
a) needn’t have done
b) must not have done
c) shouldn’t have done
d) can not have done
"needn’t +不定式完成式"表示"本來不必做，但實際做了"。題意是：你沒有必要做全部計算，這類計算我們用計算機來做。故答案為a. |
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