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.