大学英语：need作情态动词时，主要用于否定句和疑问句，表示“没有必要……”或“有必要……吗？”

You needn’t come all the way just to see the film.

Need we accompany our aging parents wherever they go?

(1)needn’t have + ed分词表示虽然已完成但却是没必要的行为。

You needn’t have paid the admission fee,for the lecture was free for members of the club.

He needn’t have bought such a large TV set.His children would have been quite happy with a smaller one.

(2)needn’t have + ed分词与didn’t need to do sth.的区别：后者表示“不一定非干某事，而通常也并没有这样做”。

I needn’t have bought the cheese,for there is plenty at home.

(我本来是根本没必要买奶酪的，家里还有很多。)

I didn’t need to buy any cheese,for in my home no one enjoys it.

(我不用买奶酪，因为家里没人爱吃。)

 注意：在didn’t need to结构中，need不是情态动词而是实义动词。