第三讲 情态动词及虚拟语气
知识梳理
情态动词表示说话人的语气和情绪，本身词义不全，不能单独作谓语，必须与后面的动词原形一起构成谓语。情态动词没有人称和数的变化，有的情态动词有过去式。常见的情态动词有can, could, may, might, must, have to, shall, should, will, would, ought to, need, dare, used to等。
考点一 情态动词的基本用法
（一）．can, could的用法
规则1：表示体力或脑力(知识、技能)所产生的能力。如：
Her mother can speak French.
规则2：表示客观的可能性。如：
Anybody can make mistakes.
规则3：表示请求建议，用could 比 can语气更委婉(回答用原形)。如：
Could you wait a few days for the money?
Could you be here at eight o'clock tomorrow morning?
规则4：表示允许、许可，用could 比 can更委婉客气。如：
Could/Can I borrow your reference books?
You can smoke in the entrance hall.
规则5：否定句、疑问句和感叹句中，表示怀疑、惊异、 不相信的态度。如：
Can it be true?
You can't be serious!
规则6：表示“经过努力后终于能……”用be able to。如：
They were able to put out the fire without any help from the firefighters.
规则7：惯用形式“cannot (can't)…too/over/enough”。
表示“无论怎么……也不(过分)”，用来加强语气。cannot/couldn’t but do sth. 不得不；只好。如：
You cannot be too careful.
（二）．may, might的用法
规则1：表示允许、请求。
Might I…？ 比 May I…？ 语气更为委婉和有礼貌。如：
—May I watch TV after supper?
—Yes, you may.或No, you may not./You'd better not./You mustn't.(强烈的禁止语气)
规则2：表示可能性，表示“或许，大概”。用于肯定句或否定句中，用might 比 may语气更加不肯定。如：
He may be very busy these days.
规则3：惯用形式 may(might) as well＋动词原形：最好，倒不如，蛮可以。如：
If that is the case, we may as well try.
规则4：表示祝愿(不用might)。采用部分倒装语序：
May＋主语＋动词原形＋……！如：
May you succeed!
May the friendship between our two peoples last forever!
（三）．must，have to的用法
规则1：must 表示“必须”。强调主观看法，只有现在时形式，否定式是 must not (mustn't)， 表示 “禁止，不准”。 如：
Everybody must obey the rules.
You mustn't speak like that to your mother.
—Must I be home before eight o'clock?
—Yes, you must./—No, you needn't./No, you don't have to.
规则2：must表示有把握的推测，意为“一定，肯定”，用于肯定句中。如：
You must be hungry after the long walk.
Home cooking must be more delicious.
规则3：have to表示“必须，不得不”，着重强调客观需要，能用于更多时态(过去式和将来式)。如：
The students today will have to know how to use computers.
As he had broken his leg, he had to lie in bed.
规则4：must 有“偏要、硬要”之意。如：
—How old are you, madam?
—If you must know, I'm twice my son's age.
（四）．will，would的用法
规则1：用于各种人称，表示意志或决心。will指现在，would则指过去。如：
I will do my best to help you.
They said that they would help us.
规则2：用于第二人称的疑问句中表示有礼貌的询问和请求，would 比 will更委婉。如：
Would you teach us how to drive a car?
Will you please give him a message when you see him?
规则3：表示习惯性、经常性、倾向性，意为“总是，惯于”。 will指现在，would指过去。如：
Fish will die without water.
He will sit for hours reading.
He would come to see me on Sunday when he was in Beijing.
规则4：表示说话人的推测，意为“大概，也许”。
would 的肯定性不如will强，语气比较弱。如：
That will be the man you want to see.
Perhaps she would be willing to meet us.
规则5：表示事物的固有属性或必然趋势，译作“总是”或“就是”。如：
The machine won't work.
He tried the door again, but it wouldn't open.
规则6：would 与used to 的区别：
(1)used to 表示过去的某种习惯，现在已经没有那样的习惯了，侧重现在与过去的对比；would 只表示过去有某种习惯，没有侧重现在与过去的对比。
(2)表示过去的状态，只能用used to，不能用would。如：
There used to be a park here.
（五）．shall, should, ought to的用法
规则1：表示征询意见，用于第一、第三人称疑问句。如：
Shall I get you some tea？我给你点儿茶好吗？
温馨提示 Shall I/we…？的回答，可用Yes, please./All right./I'm sorry, but…等。
规则2：表示说话人的意愿，有“命令，警告；允诺，承诺”等意思，用于第二、第三人称陈述句。如：
You shall do as I say.按我说的做。(命令)
You shall have my answer tomorrow.
你明天可以得到我的答复。(允诺)
规则3：shall用在条约，规章，法令等文件中，表示必然结果，多用于第二、三人称，常译为“必须”。如：
“The interest shall be divided into five parts, according to the agreement made by both sides.” declared the judge. 法官宣判，“利润应按照双方协定分成五份。”
温馨提示 must表示“必须”，只是一种主观要求，语气与shall相比，差之甚远。
规则4：should表义务，意为“应该”，可用于各种人称。如：
You should be polite to the old.你对老人应该有礼貌。
规则5：should表示推测或责备，意为“想必一定，照说应该，估计”等；表推测时往往指推测有一定的依据。如：
The film should be very good as it is starring firstclass actors.这部电影是一流演员主演的，估计拍得很好。
It's 8 o'clock now. They should be home now.现在是8点了。他们应该在家。
规则6：should用于表示惊奇、遗憾不该发生的事情，常译作“竟然”。如：
Why should you be so late today?
你今天竟然这么晚？
规则7：在大多数情况下，ought to都可以被should代替。ought to语气比should重，往往表示从法律上或从道义上“应该”。如：
You shouldn't judge a stranger always by the clothes he wears.你不应当总是以貌取人。
Parents ought to send their children to school when they reach seven years old.
当孩子满七岁时，家长应该送孩子去上学。
（六）．need的用法
规则1：need作情态动词表示“需要，必要”。通常用于否定句、疑问句、条件句中，且只有现在时，其他时态用“have to”的相应形式代替。如：
You needn't water the tomato plants now.
—Need he come now?
—Yes, he must./No, he needn't/he doesn't have to.
规则2: need还可以用作行为动词，其变化和一般的动词相同。若主语为动作承受者时用动词主动形式表示被动意义或用不定式的被动形式。如：
We need to have a rest.
The house needs repairing.＝The house needs to be repaired.
规则3：“Must…？”一般疑问句的否定回答要用：
No，sb. needn't/don't have to等。如：
—Must I finish my homework now?
—No，you needn't/don't have to.
（七）．dare的用法
规则1：dare作情态动词表示“敢于”，用于否定句、疑问句和条件句中。如：
She dare not go there.
How dare he do such a thing?
规则2：惯用短语“I dare say”意为“我想，大概”。
如：I dare say he is right.
规则3：dare还可以用作行为动词，其变化与一般动词相同。如：
The girl didn't dare to go home.
Do you dare to jump into the ocean?
I don't dare (to) ask her.
温馨提示
在否定句中，dare后的“to＋动词原形”可以省略to。
考点二 情态动词的其他用法
can/must/may/should这4个情态动词可以用来对现在的情况、过去已经发生的动作或将来发生的行为进行推测，或表示现在、过去的一种可能性。
1．情态动词＋动词原形
规则1：can 用于否定句和疑问句中，表示对现在状态、现在正在进行的动作的推测。
(1)用于否定句，意为：“不可能”。如：
He is in hospital. He can't be at school.
(2)用于疑问句，意为：“会不会”。如：
Can he be free now?
规则2：may 用于肯定句和否定句中，表示对现在的状态、现在正在进行的动作的推测。
(1)用于肯定句，意为“也许，可能”(might 的可能性比may更小)。如：
He has a car. He may come by car, but I am not sure.
(2)用于否定句，意为：“也许不/没有，可能不/没有”。如：It is raining so hard. My friend may not come.
规则3：must 表示猜测时，常与be连用，只能用于肯定句，意为：“肯定，必定”。如：
The baby doesn't want anything. She must be full.
规则4：should/ought to表示猜测时，只能用于肯定句，意为“理应，应当”(依据常规、常理、风俗、习惯等进行推测)。如：
The new coat ought to be ready on Thursday.
It's 9：00. The supermarket should be open.
2．情态动词＋完成式
规则1：must have done 表示对过去所发生的事情的肯定推测，意为“一定做过某事”，如：
There's no light in the room. They must have gone to bed.
规则2：can‘t/cannot have done 表示过去所发生行为的不可能性，通常用在否定句和疑问句中。
could have done 表示过去本可能发生而实际上未发生的事情，意为“本可以”。如：
You could have done the work better.
你本来可以把工作做得更好一些的。
温馨提示
(1)当must作“必须”意义讲时，其反意疑问部分用needn't;_当含有mustn't时，其反意疑问部分用must/may。如：
You must go now, needn't you?
You mustn't smoke here, must/may you?
(2)当情态动词must表推测时，反意疑问部分助动词的使用要根据情态动词后面所隐含的时间来确定，
如：You must be hungry now, aren't you?
You must have heard about it, haven't you?
You must have watched that football match last night, didn't you?
考点三 虚拟语气
1．虚拟语气在条件句中的用法
规则1：条件与现在事实相反， 从句谓语用一般过去时(be动词用were)； 主句谓语用would/should/could/might ＋动词原形。
规则2：条件与过去事实相反， 从句谓语用过去完成时had done； 主句谓语用would/should/could/might＋ have done。
如： If I had done it in time, I should have had a good time. 如果我及时做的话，日子就好过多了。(可惜当时没能及时做)
规则3：条件与将来事实相反，从句谓语用should do/were to do或一般过去时；主句谓语用would/should/could/might＋动词原形。
如：If it were to rain tomorrow, I should not drive my car.
1． 如果明天下雨的话，我就不开车了。(明天的情况还不知道)
规则4：主句中的should通常用于第一人称，would可用于任何人称。
规则5：当条件从句的行为与主句所表示的行为所发生的时间不一致时，主从句中动词的形式要根据它所表示的时间作出相应的调整，这就是所谓的“错综条件虚拟语气”。
如：If they had studied hard, they could do it easily now.
如果他们以前努力学习的话，现在做得就会容易些了。
If he had not taken my advice, he wouldn't do it much better like this.
如果他过去不听我的建议，他就不会做得这么好了。
规则6：在条件句中，如果有were, had, should等，则可省略if，但应注意把were, had, should等提到从句主语之前。
如：If he were to come, I would join him in the discussion. ＝ Were he to come, I would join him in the discussion. 如果他来，我将和他一道参加讨论。
规则7：有些虚拟条件句没有从句，虚拟条件句是通过上下文或介词短语表示出来的。
如：But for air and water, there would be no life on the earth.
要是没有空气和水，地球上就没有生命。(介词短语butfor提供了虚拟条件)
With your help, we might finish the plan earlier. 要是有你的帮助，我们就可以早些完成任务。(介词短语with your help相当于虚拟条件句)
2. 虚拟语气在wish从句中的用法
规则1：表示愿望与“现在事实相反”，从句用一般过去时。如：
I wish I were you.
规则2：表示愿望与“过去事实相反”，从句用过去完成时。
如：I wish I had seen the film last night.
规则3：表示愿望实现的可能性很小，从句常用could/might/would＋动词原形。
如：We wish we would live on the moon one day.
3. 虚拟语气在名词性从句中的使用
规则1：在一个坚持insist；三个命令command，order, instruct；四个建议advise, propose, suggest，recommend ；五个要求demand， desire， request，require，ask，等表示“坚持、命令、建议、要求”的动词后接的宾语从句中要使用虚拟语气。
如：He proposed that we (should) deal with the problem by the view of development. 他建议我们应该用发展的眼光处理这个问题。
规则2：与上述动词相对应的名词suggestion, order, demand, proposal 等后的表语从句、同位语从句中也要使用虚拟语气(should可省略)。
如：My proposal is that we (should) set a deadline for handing in the plan.
我的建议是为这个计划设定一个上交的期限。
规则3：在It be suggested (ordered, demanded, proposed，…)that…结构中，主语从句中也要使用虚拟语气。
如：It's required that every student be on time for school.
要求每个学生准时到校。
规则4：do you suggest/recommend用在特殊疑问句中作插入语时，句子的动词也使用虚拟语气，即“should＋动词原形”的形式，其中should可以省略。
如：What type of computer do you recommend/suggest we (should) buy?
你建议我们买什么类型的电脑呢？
规则5：在It's＋necessary，essential，important, strange, natural等形容词＋that从句或It's a pity，a shame等名词＋that 从句中，谓语动词可以使用should do。
如：It is necessary that the badly wounded man should be treated immediately. 这位重伤员必须马上治疗。
It's a pity that you should be so careless.你竟然如此粗心，真是可惜。
4. 虚拟语气在状语从句中的用法
规则1: so that/in order that目的状语从句： 从句can/could/may/might＋动词原形。
规则2: for fear that/in case that 目的状语从句：从句用should＋动词原形，意为“以防，万一”。
规则3：as if/as though方式状语从句：从句的虚拟语气与wish 后的宾语从句虚拟语气类似。
如：I've loved you as if you were my relative.
我一直爱你仿佛你是我的亲人。
5.虚拟语气在其他一些句型中的用法
规则1： It's high time that从句中，谓语动词可以使用过去式，也可使用should＋动词原形；表示“早该……了”，其中should不可省略。
如：It's high time we got up/should get up.
我们早该起床了。
规则2：would rather＋从句中，谓语动词用过去式表示与现在或将来的情况相反；用过去完成式，表示与过去的情况相反。
如：I would rather you came tomorrow. 我宁愿你明天来。
I would rather it had rained yesterday.我宁愿昨天下雨了。
规则3：if only+从句 对现在的虚拟，从句用一般过去时；对过去的虚拟，从句用过去完成时；对将来的虚拟，从句用过去将来时。“但愿，要是……多好啊。”
如：If only you hadn't offended him. 你当时不惹他就好了。
If only he could come tomorrow. 他明天能来就好了。
课堂巩固练习
1. I should not have laughed if I________you were serious.
A．thought B．would think C．had thought D．have thought
2. I________to my cousin’s birthday party last night，but I was not available.
A．went B．had gone C．would go D．would have gone
3. It ________be the vocabulary that caused you the problem in the exercise because you know a lot of words.
A．may B．couldn’t
C．should D．needn’t
4. If we________a table earlier，we wouldn’t be standing here in a queue.
A．have booked B．booked
C．book D．had booked
5. —You needn’t take an umbrella.It isn’t going to rain.
—Well，I don’t know.It________do.
A．might B．need
C．would D．should
6．Sorry，Cathy，I didn’t know that you were badly short of money then.But you ________ me for help.
A．must have asked
B．could have asked
C．would have asked
D．may have asked
7．—I called you at 9 last night，but no one answered.
—Sorry，I ________ sleeping.I went to bed very early.
A．need have been B．would have been
C．must have been D．should have been
8．I’m as busy as a bee；________ you bother me with so many questions now?
A．may B．must
C．can D．shall
9．—Did you go to the fashion show last week?
—I ________，but I was fully occupied the whole week.
A．must go B．must have gone
C．would go D．would have gone
10．It is important to know about the cultural differences that ________ cause problems.
A．must B．dare
C．may D．shall
课后自我检测
1．It is required by the rules that students ________ get grades not lower than 85 in any subject in order to get the scholarship.
A．will B．may
C．shall D．would
2．I’m really very busy，otherwise I ________ certainly go there with you.
A．can B．shall
C．will D．would
3．—God！My cell phone is missing.
—You ________ have left it at home.I heard you telephone our daughter in the bathroom.
A．shall B．must
C．would D．could
4．Even students of average intelligence ________ become top students by improving their study habits.
A．must B．can
C．have to D．ought to
5．—Look，I’ve bought some bread，including your favorite flavor.
—You ________ any because we still have plenty at home.
A．should have bought
B．could have bought
C．needn’t have bought
D．mustn’t have bought
6．It is beyond my understanding that many adults ________ be so crazy about Harry Potter series.
A．will B．can
C．may D．should
7．It’s a pity that Andrew didn’t want to go to the conference.________ willing to go，we could have paid all his expenses.
A．Had he been B．Being
C．Was he D．He had been
8．These days some young people just ________ work hard because they have got used to the easy life provided by their parents.
A．mustn’t B．won’t
C．mightn’t D．shouldn’t
9．—I regret to say that I failed to rank first in the terminal exam.
—Come on，nobody ________ win all the time.
A．should B．must
C．shall D．can
10．—May I take this book out of the reading room?
—No，you ________.You read it in here.
A．mightn’t B．won’t
C．needn’t D．mustn’t
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