（1）we mustn't buy things we （don't need）和No,I （needn't ）这两个可以互换吗?为什么.
（2）she didn't have to find them .They found her 这里的didn't have to 和needn't have 互换吧.那这句话的言外之意是什么.是她已经去找过了但其实不必要的?
（3）（不必要可以用needn't和don't have to 表示.它的完成时和过去式分别是needn't fave,didn't have to和didn'tneed to） 这句话就是说前面的那个完成时表示的是已经做了,但没有必要.那后面的过去式又表示什么意思呢