请问should,could,would的分别和用法?这个三个词是shall,can,will的过去式吧?（sorry,英语不是很好-_-|||)它们的区别在哪里?还有用法又是怎样呢?

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1 情态动词的语法特征
1） 情态动词不能表示正在发生或已经发生的事情,只表示期待或估计某事的发生.
2） 情态动词 除ought 和have 外,后面只能接不带to 的不定式.
3） 情态动词没有人称,数的变化,即情态动词第三人称单数不加-s.
4） 情态动词没有非谓语形式,即没有不定式,分词,等形式.
2 比较can 和be able to
1）can could 表示能力；可能 (过去时用could),
只用于现在式和过去式(could).be able to可以用于各种时态.
They will be able to tell you the news soon. 他很快就能告诉你消息了.
2）只用be able to
a. 位于助动词后.
b. 情态动词后.
c. 表示过去某时刻动作时.
d. 用于句首表示条件.
e. 表示成功地做了某事时,只能用was/were able to, 不能用could.
He was able to flee Europe before the war broke out.
= He managed to flee Europe before the war broke out.
注意：could不表示时态
1）提出委婉的请求,（注意在回答中不可用could）.
--- Could I have the television on?
--- Yes, you can. / No, you can't.
2）在否定,疑问句中表示推测或怀疑.
He couldn't be a bad man.
他不大可能是坏人.
3 比较may和might
1） 表示允许或请求；表示没有把握的推测；may 放在句首,表示祝愿.
May God bless you!
He might be at home.
注意： might 表示推测时,不表示时态.只是可能性比may 小.
2） 成语： may/might as well,后面接不带to 的不定式,意为"不妨".
If that is the case, we may as well try.
典型例题
Peter ___come with us tonight, but he isn't very sure yet.
A. must B. may C. can D. will
答案B. 表可能性只能用may. 此句意可从后半句推出.
4 比较have to和must
1) 两词都是'必须'的意思,have to 表示客观的需要, must 表示说话人主观上的看法,既主观上的必要.
My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厉害,我只得半夜里把医生请来.(客观上需要做这件事)
He said that they must work hard. 他说他们必须努力工作.（主观上要做这件事）
2) have to有人称、数、时态的变化,而must只有一种形式.但must 可用于间接引语中表示过去的必要或义务.
He had to look after his sister yesterday.
3） 在否定结构中： don't have to 表示"不必"
mustn't 表示"禁止",
You don't have to tell him about it. 你不一定要把此事告诉他.
You mustn't tell him about it. 你一定不要把这件事告诉他.
5 must表示推测
1) must用在肯定句中表示较有把握的推测,意为"一定".
2) must表对现在的状态或现在正发生的事情的推测时, must 后面通常接系动词be 的原形或行为动词的进行式.
You have worked hard all day.You must be tired. 你辛苦干一整天,一定累了.（对现在情况的推测判断）
He must be working in his office. 他一定在办公室工作呢.
比较：
He must be staying there.
他现在肯定呆在那里.
He must stay there.
他必须呆在那.
3) must 表示对已发生的事情的推测时,must 要接完成式.
I didn't hear the phone. I must have been asleep. 我刚才没有听到电话,我想必是睡着了.
4) must表示对过去某时正发生的事情的推测,must 后面要接不定式的完成进行式.
---Why didn't you answer my phone call?
---Well, I must have been sleeping, so I didn't hear it.
5) 否定推测用can't.
If Tom didn't leave here until five o'clock, he can't be home yet. 如果汤姆五点才离开这儿,他此时一定还未到家.
6 表示推测的用法
can, could, may, might, must 皆可表示推测,其用法如下：
1）情态动词+动词原形.
表示对现在或将来的情况的推测,此时动词通常为系动词.
I don't know where she is, she may be in Wuhan.
2）情态动词+动词现在进行时.
表示对现在或将来正在进行的情况进行推测.
At this moment, our teacher must be correcting our exam papers.
这时,我们老师想必在批改试卷.
3）情态动词+动词完成时.
表示对过去情况的推测.
We would have finished this work by the end of next December.
明年十二月底前我们很可能已完成这项工作了.
The road is wet. It must have rained last night.
地是湿的,昨天晚上一定下雨了.
4）情态动词+动词的现在完成进行时.
表示对过去正在发生事情的推测.
Your mother must have been looking for you.
你妈妈一定一直在找你.
5）推测的否定形式,疑问形式用can't, couldn't表示.
Mike can't have found his car, for he came to work by bus this morning.
迈克一定还没有找回他的车,因为早上他是坐公共汽车来上班的.
注意：could, might表示推测时不表示时态,其推测的程度不如can, may.
7 情态动词+ have +过去分词
1) may(might) have + done sth, can (could) have + done sth 表示过去,推测过去时间里可能发生的事情.
Philip may (might) have been hurt seriously in the car accident.
Philip can (could) have been hurt seriously in the car accident.
2) must have +done sth,对过去时间里可能发生的事情的推测,语气较强,具有"肯定","谅必"的意思.
---Linda has gone to work, but her bicycle is still here.
---She must have gone by bus.
3) ought to have done sth, should have done sth
本应该做某事,而事实上并没有做.否定句表示"不该做某事而做了".
You ought to (should) have been more careful in this experiment.
He ought not to have thrown the old clothes away.（事实上已扔了.）
ought to 在语气上比should 要强.
4) needn't have done sth 本没必要做某事
I dressed very warmly for the trip, but I needn't have done so. The weather was hot.
5) would like to have done sth 本打算做某事
I would like to have read the article, but I was very busy then.
8 should 和ought to
should 和ought to 都为"应该"的意思,可用于各种人称.
---Ought he to go?
---Yes. I think he ought to.
表示要求,命令时,语气由 should（应该）、had better最好）、must（必须）渐强.
9 had better表示"最好"
had better 相当于一个助动词,它只有一种形式,它后面要跟动词原形.
had better do sth
had better not do sth
It is pretty cold. You'd better put on my coat.
She'd better not play with the dog.
had better have done sth表示与事实相反的结果,意为"本来最好".
You had better have come earlier.
10 would rather表示"宁愿"
would rather do
would rather not do
would rather… than… 宁愿……而不愿.
还有would sooner, had rather, had sooner都表示"宁愿"、"宁可"的意思.
If I have a choice, I had sooner not continue my studies at this school.
I would rather stay here than go home. = I would stay here rather than go home.
典型例题
----Shall we go skating or stay at home?
----Which ___ do?
A. do you rather B. would you rather C. will you rather D. should you rather
答案B.本题考查情态动词rather的用法,would rather +do sth 意为"宁愿",本题为疑问句,would 提前,所以选B.
11 will和would
注意：
1）would like； Would like to do = want to 想要,为固定搭配.
Would you like to go with me?
2）Will you…? Would you like…? 表示肯定含义的请求劝说时,疑问句中一般用some, 而不是any.
Would you like some cake?
3）否定结构中用will,一般不用would, won't you是一种委婉语气.
Won't you sit down?
12 情态动词的回答方式
问句 肯定回答 否定回答
Need you…? Yes, I must. No,I needn't
Must you…? /don't have to.
典型例题
1）---Could I borrow your dictionary?
---Yes, of course, you____.
A. might B. will C. can D. should
答案C.could表示委婉的语气,并不为时态.答语中of course,表示肯定的语气,允许某人做某事时,用can和 may来表达,不能用could或might.复习： will 与you连用,用来提出要求或下命令.should与you 连用,用来提出劝告.
2）---Shall I tell John about it?
---No, you ___. I've told him already.
A. needn't B. wouldn't C. mustn't D. shouldn't
答案A.needn't 不必,不用. wouldn't 将不, 不会的. mustn't 禁止、不能. shouldn't 不应该.本题为不需要,不必的意思,应用needn't.
3）---Don't forget to come to my birthday party tomorrow.
---______.
A. I don't B. I won't C. I can't D. I haven't
答案B. will既可当作情态动词,表请求、建议、也可作为实义动词表"意愿、意志、决心",本题表示决心,选B.
13 带to 的情态动词
带to 的情态动词有四个：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to,为六个.它们的疑问,否定形式应予以注意：
Do they have to pay their bill before the end of the month?
She didn't use to play tennis before she was fourteen.
You ought not to have told her all about it.
Ought he to see a heart specialist at once.?
ought to 本身作为情态动词使用.其他的词作为实义动词使用,变疑问,否定时,须有do 等助动词协助.
典型例题
Tom ought not to ___ me your secret, but he meant no harm.
A.have told B.tell C.be telling D. having told
答案A.由于后句为过去时,告诉秘密的动作又发生在其前因,此地应用过去完成时,但它在情态动词 ought to 后,所以用have.
14 比较need和dare
这两词既可做实义动词用,又可做情态动词用.作为情态动词,两者都只能用于疑问句,否定句和条件句. need 作实义动词时后面的不定式必须带to,而dare作实义动词用时, 后面的to 时常可以被省略.
1) 实义动词： need （需要, 要求）
need + n. / to do sth
2) 情态动词： need,只用原形need后加do,否定形式为need not.
Need you go yet?
Yes, I must. / No, I needn't.
3) need 的被动含义：need, want, require, worth（形容词）后面接doing也可以表示被动：
need doing = need to be done