1 情况动词must 和词组have to, need to ,have got to 都表示必须做某事或者要求做某事。
need to
have to
must
have got to
When is the project due?
We need to(have to, must, have got to) have it by Monday.
2 情态动词询问和谈论过去必要事情的方式
Did Rupert need to attend last month‘s conference in Munich?
Yes, he needed to(had to) attend the conference.
3 情态动词的否定式表示不必要或不要求的事情。主要现在是和过去时的格式
You don‘t need to(haven‘t got to, don‘t have to) finish this report until tomorrow.
You didn‘t have to(didn‘t need to) finish this report yesterday.
4 词组must的否定式must not/mustn‘t 表示不允许做某事
You mustn‘t smoke in the hall!
原文：http://www.cnblogs.com/Latiny/p/7771410.html