must....should....can...need的区别，各什么时候用。

意思不太一样啊，
must是必须的意思，带命令的语气；should是应该，表示建议；can是能够，表能力但不一定是事实，need是需要。
我写几个例句吧：You must finish your homework before you go to play.你必须完成作业后才能出去玩。
You should tell him the truth.你应该告诉他实情。
You can do it.你能够做到。
We need light to read at night.晚上看书需要灯。
回答者：小Hey - 魔法师 四级 8-11 20:26

这四个词都是情态动词，具体区别如下：
must：
1）表示必须必要
You must come in time.
回答must引出的问句时，如果是否定的，不能用mustn't，而是要用needn't或don't have to。如
——Must we hand in our exercise books today？
——Yes, you must.
——No, you don't have to (you needn't).
2)must+be+表语的结构表示有把握的推测，他的否定或疑问形式用can代替must。如：
This must be your pen.

should:
1)表示劝告、建议、命令， 其同义词是ought to.在疑问句中，通常用should 代替ought to。如
You should go to class right away.
Should I open the window?
2)表示应该做，且有一种道义上的责任。如
I should help him because he is in trouble.
3)表示明显的结果或合理的推断，且所期待的是几乎是事实，因此不是虚拟语气。
Mary took dancing lessons for years; she should be an excellent dancer.
It is three o'clock, the football game should begin soon.
4）疑问句中，should表示不合理，难以相信或不应该的事，意为“竟会”，常与how, why, who 等联用。如
Why should he publish such a book?
5)用在I'm surprised ;You can't imagine 等结构中，表示惊讶，忧虑，惋惜等情绪。如：
You can't imagine that a well-behaved gentleman should be so rude to a lady.

can:
1)表示能力，如
Can you finish this work tonight?
2)表示请求和允许。如
Can I go now?
3)表示可能性，如
We can call for you at nine.
4)表示推测某一事物某一时的情况，如
Our house is on the top of the hill, and in winter the winds can be pretty cold.
5)表示惊异、怀疑、不相信的态度（主要用在否定句、疑问句或感叹句中）如
Can this be true?

need:
1)表示“需要”或“必须”，做情态动词用时，仅用于否定句或疑问句。在肯定句中一般用must, have to, ought to 或should 代替。如
-Need I finish the work today?
-Yes, you must.
2)作实意动词时，又人称、时态和数的变化。
They did not need to arrive so early.