情态动词的用法：must,can(could),may(might),ought to need,dare shall(

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.
can用于表示“能力”时是指现在的能力,过去或将来的能力通常用was/were able to或will/shall be able to表示.
eg. She was able to go to school yesterday.
Soon she will be able to swim a quarter of a mile.
Can的过去式形式could也可用于表示泛指过去的能力.如： I could read when I was four.
但不能用于表示特定的某一过去能力,如不能说：
*He could swim halfway before he got tired.
He was able to swim halfway before he got tired. 上述这种用法的区别不存在于否定句中.在否定句中,couldn’t与was/were not able to 可以互换使用. eg. I couldn’t see him again before he left.
I wasn’t able to see him again before he left. 另外,要说一个阶段延续至今的能力,可用have/has been able to表示. eg. John has been able to swim for many years.
注意的一点是：can也可以用于表示“许可”.它可以表示现在许可或将来许可,过去许可用could表示,此时一律不能用be able to的某种形式替换.

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.
3) may和might可以表示可能性和许可.might的语气比may较婉转.此外,may也可以接动词的完成形式,表示可能、说不定已经做某事.
【例如】
I thought it might be difficult for me to learn advanced mathematics. (可能性)
May I come in, sir? (许可)
She may have gone to the hospital.

4 比较have to和must
1）must 表示“必须要做的事”,否定式用 do not need to或need not做 “不必”,“不要”讲；若用must not （或mustn‘t）,则作“不可以”,“不许”,“不要”讲,有“禁止”的含义.
例 1 We must think carefully before taking actions. 在采取行动以前,我们必须仔细考虑.
例 2 —Must I finish all assignments at a time? 我必须一次完成所有的任务吗?
—No, you needn‘t. 不,不必.
例 3 You mustn’t get down while the car is still moving. 车未停稳,不可下车.
2） must 作“准是”,“一定”讲时,表示人们对某人某事的猜测,一般用于肯定句中,由于推论的某事所处时间不同,must 后的动词形式也各不相同.
例 1 You must be the new teacher. 你准是新教师.（一般现在时）
例 2 He must be joking. 他准是在开玩笑.（现在进行时）
例 3 There is nobody here. They must have all gone home. 这儿没人,他们准是都回家了.（现在完成时）
备注 must 表示猜测,推论时,只能用在肯定句中,不能用在疑问句和否定句中,此时可以用can,can’t 或could,couldn’t 来代替.
3） 在表示“必须”这个意思时,must 和have to 稍有区别.must着重说明主观看法,have to 强调客观需要.若表示命令只能用must.另外,have to 能用于更多时态.
例 1 You must leave now before making more troubles. 趁你还没惹出更多的麻烦你现在必须离开.（主观原因）
例 2 It’s so dark I have to go home. 天黑了我得回家.（客观原因）
例 3 You had to arrive before the game began. 你必须在游戏开始之前达到.（客观原因）
例 4 We’ll have to tell him all about the matter. 我们将不得不告知他全部事情.（客观原因）
注 1 have got to 可代替have to,但一般只用于现在一般时态中.
例如 I have got to （=have to） send for the lawyer. 我得派人请律师.
注 2 美式英语中,have to 的疑问式和否定式要用助动词do帮助构成,而do后面不能用have got to 的形式.
例 1 We don’t have to talk about this problem any more. 我们不必再谈论这个问题.
例 2 She has to type this letter, doesn’t she? 她得打这封信,是吗?

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
英语中,表示对过去某种情况进行推测或对已发生的动作或存在的状态进行推测,常用情态动词must, may, might, can, could, should, ought to, would等＋have＋过去分词表示.一般来说,说话人的语气或推测的把握程度不同,使用的情态动词也不同.现分述如下：
1. must＋have＋过去分词：表示对过去发生的动作或存在的状态进行推测.只用于肯定的陈述句中(在否定句及疑问句中用can ),意思是"一定已经".例如：
If he had really been there, I must have seen him.如果他当时确实在那里,我一定会看见他.
Her watch must have stopped. I will go and call her.她的表准是停了.我去叫她.
2. may／might＋have＋过去分词：表示对过去发生的动作或存在的状态进行不肯定的推测,即说话人认为过去某事发生的可能性或某状态存在的可能性一般.可用于肯定句(意思是"可能已经")和否定句(意思是"可能没有")；might的语气没有may那么肯定.例如：
She may have read the book.她可能已经看过这本书了.
He might not have got your letter.他或许没有收到你的来信.
3. can／could＋have＋过去分词：表示对过去某种行为或存在的状态的怀疑或不肯定推测.can常用于否定句(意思是"不可能已经")和疑问句；could除用于否定句及疑问句外,还可用于肯定句(意思是"那时可能;本来可以").例如：
Don‘t worry-they could have just forgotten to phone.别担心--他们很可能只是忘了打电话.
Where can he have gone?他可能去哪里了呢?
4. need＋have＋过去分词：该句式常用于否定句,表示过去做了不必做或不需要做的事情或过去做某事纯属多余,含有"根本不必；本来不必"之意,也可用于疑问句.例如：
You needn‘t have told them that.你没有必要告诉他们那个.
Need they have done it last week?他们上周做这事有必要吗?
5. should／ought to＋have＋过去分词：肯定式表示"过去本应做某事(但没做)"；否定式表示"过去不该做某事(但做了)".该句式含有不满或责备之意.例如：
You‘re right. I should have thought of that.你说得对.我应当想到那点(但没想到).
She ought to have been more careful.她本该更小心些.
6. would＋have＋过去分词：表示过去本来要做某事却因某种原因未做,通常用来说明某种情况,但不像用should或ought to那样含有责备之意.例如：
I would have written to you earlier but I have been ill.我本来要早点给你写信,但我生病了.
If I had seen the advertisement I would have applied for the job.我要是看见那个广告了,我就申请那份工作了.
7．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.
1) should和ought to表示责任、建议或劝告,翻译为“应该”,后者语气强一些.
【例如】
You should listen to the doctor’s advice if you want to recover soon.
Such things ought not to be allowed.
2) should接动词的完成式,是一种谴责,表示过去应该做而没有做或过去做了而不应该做.
【例如】
I missed the class. I should have come earlier.
I shouldn’t have made such a foolish mistake. 我,不是作家,不能用手中的钢笔描写生动的生活；我,不是画师,不会用手中的彩笔构画精彩的生活；我,更不是智者,用自己独具的慧眼来看透生活；我,只是个无知的孩子,正试图用迷惑的眼睛读懂生活……
8 had better表示"最好"
had better(常简略为‘d better)是一固定词组,had better"最好",用于表示对别人的劝告、建议或表示一种愿望.其用法有以下几点：
一、had better后面必须跟动词原形.
had better后跟动词原形(即不带to的不定式),构成had better do sth.句型. 这里的had不能用have来替换.如：
You‘d better go to hospital at once.你最好立即去医院看病.
Tom, you‘d better go there today.汤姆,你最好今天去那儿.
二、主语不论是第几人称,句子不论是什么时态,都要用had better的形式.如：
Now you(he,we) had better listen to the teacher.你(他,我们)现在最好听老师讲.
三、had better可用于的时态.
1.had better用于指现在.如：
Now you had better listen to the radio.你现在最好听收音机.
You had better be quiet.你最好安静一些.
2.had better用于指将来.如：
You had better start tomorrow.你最好明天动身.
We had better buy the more expensive one. It will last much longer and so it will be cheaper in the end.我们最好还是买贵些的,它更经久耐用,所以到头来,它还是更便宜些.
3.had better用于进行式,表示讲话人提出建议时,动作正在进行.如：
We had better be watching the TV play.我们最好现在就看到电视剧.
We had better be starting back now.我们最好现在就动身回去.
4.had better + have + V-ed可表示"本该做而没有做的事".如：
You had better have done it .如果你把那件事办妥就好了.(但实际上你没有那样做)
You had better have stayed with us .要是当时你和我们呆在一起多好啊.(实际上没有)
四、had better的否定式.如： 常用的否定形式是将否定副词not直接放在had better的后面.
如：
You had better not miss the last bus.你最好不要错过末班公共汽车.
You had better not leave for Nanjing the day after tomorrow.你最好后天不要动身去南京.
注意：否定副词not绝不能放在had的后面.如：不能说：You hadn‘t better go.而应该说：You had better not go.
五、had better的疑问式.
had better通常不以普通疑问句的形式出现.但有时用于否定疑问句,常用Hadn‘t+主语+better...?结构,作为一种劝告形式,比肯定形式的语气更婉转一些.如：
Hadn‘t you better take an umbrella?你不认为该带一把伞吗?
Hadn‘t you better ask him first?你不认为该先问问他吗?
六、had better在反意疑问句中.
在反意疑问句中,疑问部分一般用"had+主语?",但有时可用"will you?"表示一种请求或建议.如：
You‘d better not go out today, had you? 今天你最好不要外出,好吗?
I must stay at home this afternoon. You had better come to my house, will you?今天下午我必须留在家里,你最好来我家,好吗?
七、在祈使句中,had有时可以省略.如：
Better not do it .最好别做那事.
Better not wait for them.最好不要等他们.
八、had better的被动式.
You had better do it.->It had better be done.
You had better have done it.->It had better have been done.
九、had better在间接引语中的用法.
在直接引语变为间接引语的过程中,had better同第一人称或第三人称连用时,保持不变；had better同第二人称连用时,可以保持不变,也可用advise+宾语+不定式(宾补)来表示.如：
He said, "Ann had better hurry."->He said (that) Ann had better hurry.
He said, " I had better hurry."->He said (that) he had better hurry.
He said to me," You had better hurry." ->He said (that) I had better hurry.->He advised me to hurry.
十、had better在表示对别人劝告、建议时,不宜用于与陌生人、长辈及上级的交谈中.
对长辈说话时,最好不用had better.比较有礼貌的说法是：It might be better for you...； It would be better for you.如：It might be better for you to help me,Grandpa.爷爷,您最好能帮我一下.
9 would rather表示"宁愿"
一、 “would rather+动词原形”是英语中常见的一个惯用句式（美国英语中多用had rather）.’d rather为would（或had）rather的缩写形式.would (had)在此决无“过去”之意,且无词性、时态的变化.该句式常用来表示选择的意愿,意为“宁可……；宁愿……；最好……”.例如：
He‘d rather work in the countryside. 他宁可到农村去工作.
If you‘d rather be alone, we‘ll all leave here. 如果你宁愿独自呆着,那我们都离开这儿.
You would rather stay at home and do some reading this evening. 今晚你最好呆在家里看点书.
二、 would rather与than连用,可构成另一个惯用句式,即：“would rather...than.”意为“宁可（愿）……（而）不要（愿）……；与其……不如……”.用以表达主语的意愿,强调经过选择后做其中一件事,而不愿做另一件事.例如： I would rather watch TV at home than go to the cinema. 我宁可在家看电视而不愿去看电影.
三、 在使用“would rather...than...”句型时,应注意以下几点：
1. than 后边也应接动词原形,但如该句型前后连接的两个动词相同,则than之后的那个动词可省去.例如： I‘d rather know that now than afterwards. 我宁可现在就知道这件事,而不是以后.
2. would rather...than...也可改写成 “would...rather than...”,其用法及含义仍不变.例如： Facing the enemies, our soldiers would die rather than surrender. 面对敌人,我们的战士宁死不屈.
3. 有时为了表示语气上的强调,可将rather than置于句首.例如： Rather than refuse to help you, I would borrow money from my friends. 我宁可向朋友借钱,也不愿拒绝帮助你.
4. would rather后也可跟从句,表示主语宁愿让某人干某事.这时,从句谓语动词要用虚拟语气,即用一般过去时表示现在或将来要做的事；用过去完成时表示已经做过的事或过去要做的事.例如：
I‘d rather she set out to do the work now. 我宁愿她现在就着手做这项工作.
I‘d rather you met her at the airport tomorrow morning. 我但愿你明天早上能在机场见到她.
I‘d rather you hadn‘t told him the news that day. 我真希望你那天没有把那个消息告诉他.
5. would rather后还可接动词的完成式,表示主语要做某事,而结果却事与愿违.例如： I‘d rather have left a note on her desk. 我本想留张字条在她书桌上的.（事实上没有留）
试比较 I‘d rather I left a note on her desk. 我宁可留张字条在她的书桌上.
10 will和would
一、will
1．表示意志、意愿,有“愿、要”之意.例如：
1) We will fight on until final victory is won.
2) I will do my best to help you.
3) She will not shove the heavy load on to others.
2．在疑问句中用于第二人称,表示说话人向对方提出请求或询问对方的意思,有“你…..好吗、你愿…..”.例如：
1) Will you help me with my work?
2) Will you join our discussion?
3．表示习惯、经常性、倾向性,有“惯于、总是”等意义.例如： Every morning he will have a walk along this river.
4．在if ,when ,until等连词引导的表示时间或条件的从句中要用一般时表将来,但在if 从句中有时为了表示意愿也可用will ,有“愿意”的意思.例如：
1) If you will read this article ,you will realize you are wrong.
2) If they won’t help us ,we can not finish the task within one month.
二、would
1．Will 的过去式,表示对过去时间的意志、意愿、决心,有“愿、要、偏要”的意思.例如：
1) I promised that I would do my best.
2) I told him not to go ,but he would not listen to me.
2．表示请求或个人的想法、看法,使语气婉转,有“愿、倒”等意思,例如：
1) Would you please take a seat?
2) Would you mind opening the window?
3) I would like to express my thanks to you.
3．表示过去习惯性的动作,着重动作的重复,有“总是、总会”等意思.例如： On Sundays he would come to our village and work with us.
4．表示虚拟、假设、条件等,有“要、将要、会、就会”等意思.例如
1) If you had come earlier ,you would have seen him.
2) It would be better to adopt the new method.
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?
11 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.
12 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.
used to + do："过去常常"表示过去习惯性的动作或状态,但如今已不存在.
Mother used not to be so forgetful.
Scarf used to take a walk. (过去常常散步)
have to 表示客观的需要,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)

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
4) 做情态动词时,need和dare 没有过去的形态.主要用于疑问句和否定句中,need 表示“需要”,dare 表示“敢”的意思.
5） need 和 dare 后用完成时用在否定句中,表示本来不必做而实际上已经做的事.
例 1 You needn’t have worried about your health; the doctor will care for you. 你其实不必担心你的健康,医生会照顾好你的.
例 2 I dare not have expressed my viewpoint. 我本来不敢阐述我的观点.