助动词与情态动词最主要的区别之一是,助动词本身没有词义,而情态动词则有自己的词义,能表示说话人对有关动作或状态的看法,或表示主观设想。例：
情态动词没有人称变换，后面接原形。
What have you been doing since? (构成完成进行体,本身无词义)
I am afraid I must be going. (一定要)
You may have read some account of the matter. (或许已经)
除此之外,情态动词还有如下词法和句法特征：
1) 除ought和used以外,其他情态动词后面只能接不带to的不定式。如果我们把ought
to和used to看作是固定词组的话,那么,所有情态动词无一例外地只能接不带to的不定式。例：
We used to grow beautiful roses.
I asked if he would come and repair my television set.
2)情态动词在限定动词词组中总是位居第一。例：
They need not have been punished so severely.
3)情态动词没有人称和数的变化。例：
She dare not say what she thinks.
4)情态动词没有非限定形式,即没有不定式、-ing分词和-ed分词形式。例：
Still, she needn’t have run away.
5)情态动词的“时”的形式并不是时间区别的主要标志。在不少场合,情态动词的现在时和过去时形式都可以表示现在、过去或将来时间。例：Would you mind very much if I ask you to do something? She told him he ought not to have done it.
6)情态动词之间是相互排斥的,即在一个限定动词词组中只能出现一个情态动词,但有时却可以与助动词have和be连用。例：
You should have washed the wound.
Well, you shouldn’t be reading a novel.
1. can与could
1) can可用于表示能力、可能性、允许等。例：
She can sing that song in English. /Can the news be true? /Can I smoke here?
2)(用于否定句、疑问句或感叹句中)表示猜测、怀疑或惊诧。例：
She couldn't / can't be so stupid to do that .
2) can 只有两种时态,过去式为could。
could 不仅用于过去,也可用于现在,表示委婉、客气,常用于疑问句；而be able to 有较多的时态形式。be able to 的过去式表示“设法做某事”,经过努力之后才完成的,而could 没有这个含义。例：
The fire spread through the hotel quickly, but everyone was able to get out.
2.may与 might
1)表示请求、许可、允许、可能性。例：
The librarian told her that she might return the book to the library in three days.
当回答由may 引起的问题时,否定答语要用must not,表示“不许可”、“不应该”、“不行” 。例：---May I come in? ---Yes, please. /--No, you mustn’t.
2)may (might) + have done 表示对过去发生行为的推测,含有“想必”、“也许是”的意
思。例：
He might not have settled the question.
3.must与have to
1)must表示必须、必要。例：We must protect people’s rights.
2)“must be + 表语”的结构,通常表示猜测,含有“一定”之意。(只用在肯定句中)例：
He must be an honest boy.
“must +have +过去分词”的结构,常用在肯定句中,表示对过去发生行为的推测,含有“一定”、“准是”的意思。否定和疑问句用can。例：It must have rained last night for it is so wet outside.
3)must 的否定式有两个：当回答由must引起的问题时,否定答复要用needn’t或don’t have to 表示“不必”、“无须”、“用不着”、“不一定”的意义。当表示“不应该”、“不许可”、“禁止”时,就用must not。例：—Must I go tomorrow? —Yes, please./Yes, you must.
—No, you needn’t./No, you don’t have to.
4)have to 比较强调客观需要,must着重说明主观看法。例：
I must clean the room.(主观想法)
I have to clean the room.(客观需要)
have to 能用于更多时态。例：We had to be there at ten .
have to 的否定式don’t have to do 表示“不必做……”之意。
4. ought to与should
1.ought to
1)表示“职责、义务”。例：
Your son is still young. You ought to take care of him.
2)表示推测。
注意与must表示推测时的区别：
He must be at home by now.断定他已到家)
He ought to be home by now.(不十分肯定)
This is where the oil must be.(比较直率)
This is where the oil ought to be.(比较含蓄)
这时,ought to和should可以互相换用。
3)“ought to + have + 过去分词”
a. 表示“本应该做,而实际没有做某事”,相当于should + have+过去分词。例：
You ought to have asked him (but you didn’t).
b. 表示“本不应该做,而实际做了某事”。例：
You oughtn’t to have stopped on the motor way.你本不应该在高速公路上停车的。
c.表示推论或推测某事已经发生,含有说话人不能肯定其说法是否争取的含义。例：
If he had started at nine o’clock, he ought to have been in London by eleven o’clock.
如果她是酒店出发的话,那他十一点中该到达伦敦了。
4)ought to 可用于否定句, 疑问句,其否定形式可缩略为oughtn’t to。例：
You oughtn’t to smoke so much.
Ought you to smoke so much?
5)ought to的疑问式及其回答。
---They ought to do it,oughtn’t they？
---Yes,they ought(to).
---No,they oughtn’t .
在美国英语中,ought to用于否定句和疑问句时,to可以省略。例：
Ought you smoke so much?
You oughtn’t smoke so much.
2.Should
1)should用于第一人称疑问句中询问对方的意愿,表示说话人的一种谦逊、客气、委婉的语气。例：
What should we do now?
2)表示应该、必须,常与must 换用。例：
We should (must) master a foreign language at least.
3)“should+be+表语”的结构,表示推测或惊奇。例：
They should be back by now.
4)“should+have+过去分词”的结构,
表示过去该做而实际上尚未做的动作或行为；其否定则表示发生了不应该发生的行为。其同义结构“ought to have +过去分词”,表示过去“早应该”、“本当”之意,语气较强。例：
I should have thought of that. 这一点我是应当想到的。(但没想到)
They should not have left so soon.他们不应当走得这么早。(但已走了)
should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。ought to表示“应该”时和should差不多,只是口气稍重一些, 常可互换用。ought to比should具有更强烈得道义、责任、法律或原则等意味；should指主观上感到有责任或义务去做某事,语气较婉转。例:
You ought to keep your promise.
You should apologize to him.
5. shall 与should
1)shall用于第一人称征求对方的意见。例：What shall I wear on the journey?
2) 用于第二、第三人称时表允诺、命令。例：You shall come to my office immediately .
3)should用于第一人称疑问句中询问对方的意愿,但语气较委婉温和,例：
What should we do now?
4)表示应该、必须,常与must 换用。例：
We should (must) master a foreign language at least.
5) should 的其他用法请参阅上条ought to与should 。
6. will与would
1)表示意志,决心或愿望。例：
Surely we will support all the people in the world in their struggle for peace.
If you will go,I shall go with you.
2)will表示经常性、习惯性或倾向性,would表示过去的习惯行为。例：
He will sit there hour after hour looking at the traffic go by.
He would come to see me when he was in Beijing.
3)用于主语是第二人称的疑问句中,表示对对方的请求,would的语气比will委婉。例：
Would/Will you kindly tell me the way to the station?
7. need 与dare
need 与dare作情态动词用时,一般用在否定句和疑问句中,如果用在肯定句中,多
数是实义动词,一般现在时单数第三人称要加s,并有时态变化,后接不定式时须带to。例：
Need you go so soon?
Dare you stay here alone in the night?
Need I explain once more? 需要我再解释一次吗？(情态动词)
The question needs to be discussed. 这个问题需要加以讨论。(实义动词)
She dare not tell the truth to him. 她不敢向他讲真话。(情态动词)
We should dare to think, to speak and to act. 我们应该敢想、敢说、敢干。(实义动词)
错句：She dares not go out at night.
He doesn’t need do that.
Dare she to go there？
8. 注意推测性用法中谓语动词发生的时间
如果推测的为过去的事情,情态动词后要用完成形式。例：
There must have been no one in, for nobody answered the phone.
如果推测的是正在进行的事情,情态动词后则用动词的进行形式。例：
They must be talking about something very secret.
如果推测的是现在的状态等,情态动词后则用动词原形。例：He must be badly ill. He looks so pale.
如果推测的是将来发生的事情,情态动词后同样用动词原形,这时,情态动词只能是may/might。例：
There may be a terrible storm in the following few days.
肯定的推测一般用may, should, must,它们的意思分别是“可能”,“应该”,“必定”,口气逐渐加强。
否定的推测常有两种情况：口气较弱,对否定不大有把握时,用may not “可能不”；口气比较强,对否定比较有把握时用cannot,表示“不可能”。推测的疑问句,一般只用can/could, 例：
Who can it be ? / Can it be true?
9.其它几个“情态动词+ 动词的完成形式”并不表示推测,注意意义上的区别：
could have done本来能够,例：
He didn't take part in the competition, he could have won, though.
needn't have done 本来没必要,例：
You needn't have hurried. There was plenty of time.
should/ought to have done 本来应该,例：
You should have told me earlier. I could have helped you.
shouldn't/oughtn't to have done 本来不应该,例：
You shouldn't have been following him so closely; you should have kept your distance.
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