比较：
Father sent me the book, so I didn't need to write to him for it.
父亲把那本书寄来了，所以我不必为此给他写信了。(信没写)
Father sent me the book, so I needn't have written to him for it.
父亲把那本书寄来了，所以我本不该为此再写信给他的。(信已写)
She didn't need to come.她不必来的。(实际也没来)
She needn't have come.她本来不必来的。(但却来了)
Note:
作助动词用时，need只用于疑问句中和否定句中，在肯定句中常用must, have to, should, ought to等。例如：
Need you do it right now? I needn't, but he must.
She doesn't need to be told. She has already known it.
(客观情况使得告诉她没有必要：她已经知道)
She needn't be told. We should keep it secret from her.
(主观上不愿告诉)
完成下面五道选择题目：
1. You__ Jim anything about it. It was none of his business.
A needn't have told B needn't tell C mustn't have told D mustn't tell
2.As it turned out to be a small house party, we __ so formally.
A need not have dressed up B must not have dressed up C did not need to dress up D must not dress up
3.We could__ him with a detached house when he came, but he had specifically asked for a small flt.
A provide B have provided C not provide D not have provided
4.He__ the 8:20 bus becasue he didn't leave home till 8:25.
A couldn't have caught B ought to have caught C shouldn't have caught D must not have caught
5."She must be in the dormitory now."
"No, she __ be there. I saw her in the classroom a minute ago."
A mustn't B can't C couldn't D wouldn't