1)need表示需要，主要用于否定句和疑问句。用于疑问句，肯定回答用must，否定回答用needn’t。have to表示他人或客观情况要求“必须”，也可以翻译为“不得不”，have to否定形式意思是“不一定”或“没有必要”。
【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
本句译为：你把这些东西洗了，你真是太好了。但是你没有必要洗。must, would, might后面接动词的完成式，都很少用否定式，而didn’t have to表示没有必要，符合题意，故答案为d.
2)need后面接动词的完成式，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“本来不必做，但实际做了”。题意是：你没有必要做全部计算，这类计算我们用计算机来做。故答案为a