助动词和情态动词

助动词和情态动词
助动词包括：be (is, am, are, was, were, been, being)；have (has, had, having)；do (does, did)；shall (should), will (would)
情态动词包括：can, could, (be able to)；may, might；will ,would；dare, need, must (have to)，ought to, shall, should
" be + 动词不定式"可以表示下面几种意思:
1.命令或指示。

例如：
No one is to enter the room without permission.
The books are not to be taken out of the reading-room.
2.计划或安排。

例如：
A new bridge is to be built over the river soon.
The expedition is to start in a week's time.
3.可能。

例如：
This kind of tree is to be found in that forest.
4.应该。

例如：
Such people are to be criticized.
5.表示后来发生的事，可以用来表示一种命运。

例如：
They said good-bye, little knowing that they were never to meet again.
He was to regret the decision.
Note:
was (were) + 不定式完成式
这种结构表示“本来打算…，本来要…（而结果则没做）”。

例如：
He was to have attended the meeting, but he fell ill.
他本来要参加会议的，但是生病了。

（因此没参加）
have to 和must在表示“必须”这个意思时是很接近的，但
也有一定的区别。

1. have to 比较强调客观需要，表示因客观环境或事态促使而不得不做某事；must强调主观看法，表示主观上认为有必要做某事。

例如：
I must learn another language.
（主观想法: I want to）
I have to learn another language.
（客观需要：身为一个外交官）
You must be back before 10 o'clock.
（叮嘱或命令）
You have to be back before 10 o'clock because the train is to leave at 10:05.（客观需要）
注意下面两句的含义：
Though she didn't need to, she must go.
虽然她不用去，她却偏要去。

You don't have to come again, but you must.
你不必再来，可是你执意要来。

2. have to多表示义务或习惯动作；
must则用于表示一种重要或急迫的事情。

例如：
We have to care for the young.（义务）
She has to be at the office before eight every day. （习惯）
You must go to the manager at once, or you'll be dismissed.
务必马上去见经理，不然你会被开除的。

（急迫的事情）
3. have to可用于不同的时态；
而must一般只用于现在时（但也可以表示将来的情况，在间接引语中也可以表示过去时）。

例如：
I have to (must) leave now.
We'll have to buy another TV set.
They had to put off the sports meet because of the bad weather.
He said that the work must be finished within two weeks.
4. must还可以表示一种推断和揣测，而have to则不能。

例如：
This must be Jin's pen.
There must be something wrong with the machine.
5. "must＋动词完成式"可以用来表示对过去情况的揣测，而have to则不能。

例如：
She must have read the book sometime in the past, or she couldn't have answered the question so well.
她一定在过去某个时候读过这本书，不然，这个问题她不会回答得这么好的。

Note:
在现代英语中，have to 中不定式和疑问句式既可以按照助动词的变化规则构成，也可以按照行为动词的变化规则构成。

例如：
Have you to finish the work before supper?
Do you have to finish the work before supper?
have got to 常可同have to换用，但有时表示不同的含义。

比较：
He has to report to the headquarters every two days.（例行公事，习惯动作）
He has got to report to the headquarters every two days.（一道指示或命令，必得每天报告一次）
另外，作“有”解时，have got和have通常是可以换用的，have got更口语化，但有时表示不同的含义。

比较：The man has a blind eye.那人有一只眼瞎了。

The man has got a blue eye.那人有一只眼被打青了。

must not表示禁止，是说话人强有力的劝告或命令，意为“一定要，不准”。

因此，在回答由must引导的问题时，如果是否定的回答，表示“不必，没有必要”，不能用mustn't，而要用needn't
或don't have to.试比较：
You must not drive fast.你不能开快车。

（路险或有速度限制）You needn't drive fast.（时间充裕）
You must not tell others.（警告）
You needn't tell others.（没有必要）
needs为副词，相当于necessarily, of necessity. must needs和needs must均可表示“必须，必定，不得不”，这层意义上可以通用；但must needs还可表示“偏偏，偏要”，含有讥讽、不满的意思，而needs must则一般无这层意思。

例如：
I must needs go there now.
我现在非到那里去不可。

（可用needs must）
Needs must when the devil drives.
情势所迫不得不那样。

She must needs go away when I want her.
我正需要她时，她偏偏离开了。

（不可用needs must）
The telephone must needs ring when I went to bed.
我上床睡觉时偏偏有人来电话。

（不可用needs must）
need指主语的主观特需情况而使之必要。

need既可以作助动词，又可以作行为动词。

作助动词时，need没有人称和数的变化，后接不带to的不定式（动词原形），否定式为needn't.作行动动词时，need同别的行为动词一样，有人称和数的变化，后接带to的不定式，否定式要在前面加don't（doesn't, didn't），疑问句用do (does, did) 提问。

例如：
There need be no hurry, need there?
He need never know. (=He never needs to know.)
I need only add a few words. (= I only need to add a few words.)
比较：
Father sent me the book, so I didn't need to write to him for it.
父亲把那本书寄来了，所以我不必为此给他写信了。

（信没写）Father sent me the book, so I needn't have written to him for it.
父亲把那本书寄来了，所以我本不该为此再写信给他的。

（信已写）
She didn't need to come.她不必来的。

（实际也没来）
She needn't have come.她本来不必来的。

（但却来了）
Note:
作助动词用时，need只用于疑问句中和否定句中，在肯定句中常用must, have to, should, ought to等。

例如：
Need you do it right now? I needn't, but he must.
She doesn't need to be told. She has already known it.
（客观情况使得告诉她没有必要：她已经知道）
She needn't be told. We should keep it secret from her.
（主观上不愿告诉）
1. ought没有词形变化，通用于所有人称，可以用于现在时，过去时和将来时，同带to的不定式连用，否定式为ought not to (oughtn't)，疑问式把ought放在主语前。

例如：
He knew he ought not to tell her that.
Note:
ought to与must, have to, should的含义比较接近，但不完全相同。

ought to表示义务或责任，用以提醒某人注意其义务，或提出劝告，指出一个正确、明智的行为。

must强调的是主观愿望，指必须做什么，牵涉到说话人的权威。

have to 则表示由某种情况、环境所迫而不得不做某事。

should同ought to用法完全相同，只是ought to口气稍重一些。

比较：
You ought to respect the old.（劝告）
You must do it at once.（含有说话人的权威性）
We'll have to reconsider the matter.（情况所迫：发现有新的问题）
We should (ought to) do more for the country.（义务，责任）
表示“必然”时，ought to的语气不像must那么肯定。

比较：
This is where the gold deposits must be.
这里一定是埋藏金子的地方。

This is where the gold deposits ought to be.
这里应是埋藏金子的地方。

used to +动词原形表示一种过去的习惯、过去的例行活动或方式，一个与现在情况相反的过去状况，意为“过去常常”，否定式为used not to (usedn't to)，疑问式把used放在主语前。

be (become, get, grow) accustomed /used to ＋动名词或名词（不能加动词原形）意为“习惯于…，对…习惯”。

例如：
He usedn't to like English, but he quite likes it now.
The students soon got used to school regulations.
She didn't use(d) to come.
She usedn't to come. 她过去不常来。

Did he used to get up early?
Used he to get up early? 他过去常早起吗？
Didn't he used to get up early?
Usedn't he to get up early? 他过去不是早起吗？
Note:
used to还可以同often, never, always连用。

例如：He often used to work late at night.他过去常工作到深夜。

She always used to get up at four o'clock in the morning.
她过去总是早晨4点起床。

除在一般将来时中用于第一人称外，shall还有如下用法。

1.表示征求意见或请求指示。

例如：
Shall we meet in the evening? 晚上见好吗？
How shall I start the machine? 怎样开动这部机器？
Note:
shall只表示“愿意按对方的指示去做”，而may, might或can 表示“征求对方的同意”，意为“行不行，好不好，可以不可以。

”比较：
Shall I come in? 你要我进来吗？
May（或might, can）I come in? 我可以进来吗？
Shall I buy the dictionary? 要我买那本词典吗？
May （或might, can）I buy the dictionary?我可买那本词典吗？
Shall I go there tomorrow? 要我明天去那里吗？
May I go there tomorrow? 我明天去那里好吗？
用于第二、三人称，要重读，不可缩写、表示意图、意志、允诺、命令或必然结果等；但shall所表示的意志是说话人的意志，而非句子中主语的意志。

在法律、条约、协定等文件中，shall 表示义务、规定等。

例如：
You shall have a lot of money= I shall give you a lot of money. （意图）
你会有很多钱的。

（我会给你很多钱的）
Each citizen shall carry his identification card when traveling.（规定）
旅行时每个公民务必带上身份证、
She shall get this paper in the evening.（允诺）
He shall get what he deserves.（警告）
Death is certain to all; all shall die. 物皆会死。

（命运）
Better days shall soon follow.（预言）
will和would除在将来时态中用于第二、三人称外，还有如下用法。

1. 表示请求（这时will和would通用，而would更委婉，也用won't）。

例如：
Will you give her the letter?
你把这封信给她好吗？
Would you please tell me your telephone number?
请把你的电话号码告诉我好吗？
2. 表示习惯或倾向。

will用于一般的习惯，would用于描述过去的习惯或例行的活动。

would同used to 强调过去的习惯性动作或状态，但如今已不存在，与现在的情况形成对比，既可表示过去持续的状态，也表示过去曾经的行为。

例如：
Scarf used to be heavy smoker.
Father used not to be so forgetful.
would强调过去某种特定情况下的动作，是完全过去的事情，同现在没有联系。

would 只表示重复的动作，不表示状态。

比较：She used to get up at six in the morning.（正，重复的行为）
She would get up at six in the morning.（误）
Man used to think that the earth was flat.（正，持续状态）
Man would think that the earth was flat.（误）
would 可以表示不规则的习惯，used to则不可。

例如：
Sometimes she would take a walk in the neighboring woods.（不可用used to ）
An Englishman will show you the way in the street.
英国人在街上是会给你指路的。

（英国人一般都会这样做。

）In those days, whenever I had difficulties, I would go to Mr. Lees for help.
He used to get up at five, but now he gets up at six.
下面两句是错误的，应改用used to:
He would be very nervous when taking an exam.
Before liberation, workers would work sixteen hours a day.
3. 表示推测。

例如：
It will be Mr. Wang knocking at the door.
It would be about ten o'clock when he left home.
They will have arrived in New York by now.
Note:
will表示推测时没有must把握大，should也可以表示推测，但比will 把握略小。

其程度由低到高为might→may→could→should→ought to→would→will→must。

4. 表示命令、强迫等（只用will，通用于所有人称）。

例如：
All will arrive before 7:45.
所有人员务必在7:45之前到达。

I won't allow her to do that.
我不会让她做那件事的。

5. 表示意愿或固执坚持。

用于非人主语时，表示固有性质、倾向。

例如：
She won't lend me the money.
她不愿把钱借给我。

He is the man who will go his own way.
他是一个自行其是的人。

The window won't open.
窗子打不开。

I won't argue with you.
比较：
He will do it, whatever might happen.
不管发生什么，他都要做这件事。

（主语He坚持要做）
He shall do it, whatever might happen.
不管发生什么，他都必须做这件事。

（我们坚持要He做）
6. 表示能力（拟人化）。

例如：
The hall will seat 1000 people.
That was a strong dog. How much would it carry?
7. 表示客观事实。

例如：
Fish will die out of water.
Oil will float on water.
8. 表示意图或允诺。

例如：
I'll trouble you for the dictionary.
You will have your share.
I won't let you down.
9. 表示拒绝，用won't 。

例如：
I won't listen to your nonsense.
I urged him to do it, but he wouldn't hear of it.
10.will在if从句中的运用。

在含有if从句的主从复合句中，如果if引导的条件状语从句表示的是一般将来时或过去将来时，不可用will或would，而要用一般现在时或一般过去时代替。

但是，will可用于if从句中表示各种“愿望”，这些愿望包括“请求，意愿，拒绝，同意，允许，能够，坚持，选择，计划”等。

例如：
If you will come into the hall, the meeting will begin soon.
请到大厅里来，会议快要开始了。

（请求）
If you will make another try. I shall do everything possible to help you.
如果你原意再试一次的话，我愿尽一切可能帮助你。

（意愿）If he won't go with you, I shall ask somebody else.
如果他不肯同你一起去的话，我将另找他人。

（拒绝）
If you will agree with me. I shall tell you everything about it.
如果你同意我的观点，我将把一切都告诉你。

（同意）
If you will not come late again, I shall let you in.
如果你答应不再迟到的话，我就让你进去。

（允诺）
If anyone will find a cure to the disease, it will be a wonder.
如果有人能够治好这种病的话，那将是一个奇迹。

（能够）
If you will do it like that, you will fail.
如果你坚持要那样做的话，你会失败的、（坚持）
If you will buy bread, I shall buy beer.
如果你买面包，我就买啤酒。

（选择）
If you won't lend money to him, please let me know.
如果你不打算把钱借他的话，就让我知道。

（计划）
Note:
在下面的句子中，would有“可能”的含义：
I wouldn't dream of it.我做梦也不会想到。

That's what he would do.他会那么做的。

You wouldn't know.你不会知道的。

CAN
指人体力、智力、性格上的主观可能，也指性质上可能．
1.表示许可。

can表示许可时，是may在非正式场合的替代词。

例如：
You can borrow two books at a time from the library.
2.表示可能性。

例如：
I can't guess whose book it is.
Measles can be quite dangerous.
3.表示能力。

can表示能力时（即有某种知识和技能而能办到），可与be able to换用，但在将来时和完成时中必须用be able to结构；表示经过努力而成功地办到了某个具体的事情时，只能用be able to, 不可用can.这种用法的be able to相当于succeed in 或manage to.例如：
Can you type?
Are you able to type?
He could swim across the river even in his later years.
He was able to swim across the river even in his later years.
He was able to swim across the river when the boat sank.
I'm sure we shall be able to get you a job soon.（不可用can）
He has been able to finish the work on time.（不可用can）
After years of hard work he was able to win the prize.（不可用could）
She couldn't climb the mountain.（没有能力爬，因而也没爬）She was not able to climb the mountain.（尝试爬过，但没能爬上去）
Note:
could是can的过去式，除具有can的各种功能外，还可以用来比较委婉、客气地提出问题或陈述看法。

例如：Could you come a little earlier?
I am sorry I couldn't lend you the book now.
can和could还可以表示某人或某物一时的特点。

例如：
It can be very misty in this area.这个地区有时会大雾弥漫。

He can be very friendly at times.他有时会很友好。

（并不是一贯友好）
can / could + have + 过去分词有时相当于may / might＋have + 过去分词，意为“可能”。

can是could 的口语体，语气上较婉转。

can not have + 过去分词和could not have + 过去分词均可表示“过去不可能”。

例如：
John can / could have been seriously hurt in the accident.
Can they have missed the train?
He couldn't have been swimming all day.
肯定的推测用must,否定的推测用can not。

比较：What she says must be true.
What she says can not be true.
can be和could be后可接possible作表语。

例如：
Can it be possible?
It can't be possible.
Could it be possible that he dialed a wrong number?
possible的副词形式possibly也可同can或could连用，表示强调。

例如：
I can't possibly do it. It's against the law.
1.表示许可（正式场合）。

例如：
You may take the book home.
Candidates may not bring reference books into the examination room.
（表示说话人不许可，相当于We don't permit…）
2.表示可以做或可能发生的事。

例如：
They may not be there today.
A situation like this may / can occur from time to time.
3.表示祝愿。

例如：
May your kingdom come!
May our friendship live long.
4.用在表示目的的状语从句中。

例如：
Get up early so that we may catch the train.
比较：
They may not go swimming.（不许可）
They may not come if it rains.如果下雨，他们就可能不来。

They mustn't go swimming.他们一定不能去游泳。

（责令不得去）
Note:
might是may的过去式，在表示“可能”这个概念时，may和might 是可以换用的，但might表示较多的怀疑，可能性较小，或者表示更婉转的语气。

试比较：
Jim may lend you the money.（可能性较大）
Jim might lend you the money.（可能性较小）
Might I ask a question?（较婉转）
回答May I …?问句时，may not意为“不可以”；回答Can…？问句时，may not意为“可能不”；may not还可以表示“可以不”例如：
May I sit here? No, you may not.
Can the story be true? It may be, or may not be.可能是，也可能不是。

You may not tell her that.你可以不把那件事告诉她。

（也可以表示：你不可把那件事告诉她）。

may表示“可能”时一般不用在疑问句中，而以be likely to等结构替代。

例如：
Is he likely to win the match?
他有可能会赢这场比赛吗？
这种结构用来建议或劝说某人采取某种行动，相当于had better.
例如：
It is very late, so you may / might as well go to bed.
夜深了，还是去睡吧。

May well+动词原形意为“理应，有足够的理由”。

例如：
He may well be proud of his son. 他大可以他的儿子为荣。

She may well say so.她说得对。

1.用于疑问句或感叹句中（表示意外、惊讶等，与how, why, who连用）。

例如：
How should I know?
Why should you be worrying?
Who should come to meet us but the manager!
Why was it that the manager would go back on his words?
2.用在某些状语从句中。

例如：
He took a taxi to the station so that she should not miss the train.（也可根据需要，用might 或could）
They removed and buried all machines for fear (that) the enemy should make use of them.
3.用在I'm surprised, It worries me, It's a pity, It's unthinkable等结构后面的句子中表示惊讶、忧虑、惋惜、欢欣等情绪。

例如：I'm surprised that he should eat so little every day.
他每天吃得那么少，真使我感到惊讶。

It's a pity that she should fail in the entrance examination.
她没有考取，真是可惜。

4.表示应该做，且有一种道义上的责任。

例如：
I should help him because he is in trouble now.
You should do it because you have promised to.
比较：
would常用来解释为什么没做某事。

例如：
I would go to the football match but the weather is too bad. So I won't.
I would do it but I have something too urgent to be delayed.
5.意为“可能，该”，表示对现在情况、将来情况或过去情况的某种推测。

例如：
He should arrive at noon.他该在中午到达。

Jim should be at home.吉姆可能在家。

He should be taking a bath now.他可能正在洗澡。

They should have finished the work by tomorrow.
到明天他们就可能完成工作了。

They should have reached the town by now.
他们现在该已经到达那座小城了。

dare表示意志上的主观可能，指在勇气、胆量上可能。

dare既可用作助动词，又可用作行为动词。

作助动词用时，dare 的否定式为dare not,后接动词原形；作行为动词用时，dare的否定式是don't (doesn't. didn't) dare，后接带to的动词不定式。

例如：
She dare not (daren't) go out at night.
(= She doesn't dare to go out at night.)
He dares to say what he thinks.
Dare you touch it again?
Note:
dare＋动词原形表示现在或将来时间，dare后也可以接动词完成式指过去的情况。

例如：
Dare he have done it yesterday?
dare作行为动词用时，后接的不定式也可省略to。

例如：
She has never dared (to) tell her mother that.
How dare you (he)…?表示“愤怒、谴责”之意，不表示疑问。

例如：
How dare you call him a liar?
你怎敢把他说成是撒谎者？
dare作及物动词时用时，有“挑激”之意。

例如：
I dare him to jump over the wall. 我挑激他跳过那堵墙。

I dare say为惯用语，意为“我想，我以为”。

例如：
She will keep her word, I dare say.我想她会守信用的。

情态助动词+ 完成体
1. may和might ＋动词完成式
此结构用于推测过去的行为。

例如：
She said that he might have missed the plane.她说他可能误了航班。

If we had taken the other road, we might have arrived earlier.
2. can和could + 动词完成式
A. 表示过去能做而没做的事，有一种对过去未付诸实施的事情的惋惜。

例如：
In those circumstances we could have done better.
在那样的情况下，我们本来可以做得更好的。

（但没有做得更好）
B. 推测过去的某种行动。

Where can /could she have gone? 她可能到哪里去了呢？
The boy can't have finished reading the book so soon because it is difficult even to an adult.
这孩子不可能那么快就把这本书读完的，因为即使对一个成人来说，这本书也是相当难的。

Note:
can't和couldn't＋动词原形可以用来表示否定推断。

例如：He can't be over fifty.他不可能超过50岁。

3. must ＋动词完成式
表示对过去行为的推断，具有较大的可能性，意为“一定…，想必…”。

例如：
She must have gone through a lot.
她一定吃过很多苦。

Note:
must＋动词原形→对现在的推测，must be＋现在分词→对未来或现在正在进行的推测。

例如：
She looks happy; she must be having a good time.
It must be raining tomorrow according to the radio.
4. needn't＋动词完成式和didn't need to do.
A. needn't＋动词完成式表示一种已经做过的但并无必要的行
为。

例如：
You needn't have watered the flowers, for it is going to rain.
你本来不必浇花的，因为天要下雨了。

（但却浇了）
We needn't have told him the news because he knew it already.
B. didn't need (have) to do结构表示没有必要做某事，实际上也没有做。

例如：
I didn't need to clean the windows. My sister did it.
我不必擦窗，我妹妹擦了。

（我没有擦）
We didn't need to carry his luggage; his brother carried it himself.
5. should + 动词完成式
A.此结构的肯定式表示应该做的事而没有做，否定式表示某种行为不该发生但却发生了。

例如：
You should have given her more help.
你应该多给她一些帮助的。

（但没有给）
She shouldn't have left the hospital so soon, for she had not yet recovered.
她本不该那么早就离开医院的，因为她还没有康复。

（但离开了）
B. 表示推测，意为“可能”，但可能性较小。

例如：
He should have finished the work by now.
他现在该把工作完成了。

（推测）
I think they should have arrived by this time.
我想他们现在该到了。

C. should / would have thought意为“本以为，本认为”。

例如：
A: "Can you type?"
B: "Certainly."
A: "Well, I should have thought you wouldn't."
I should have thought she wouldn't agree.
我本来认为她不会同意的。

D. should have thought有时意为“认为，以为”，相当于should
think，但表示更为委婉、谦逊或不肯定的语气。

例如：
I should have thought it fairly good.
我以为它是很不错的。

I should have thought you might take this into consideration.
我认为你要把这个考虑进去。

E. should have ＋过去分词用于虚拟语气：
Had you written him, you should have known the details.
6. ought to + 动词完成式
此结构表示某种过去应该做而没有做的事，或被忽略未做的某种明智行为。

例如：
You ought to have returned the book earlier.（还晚了）
You ought to have refused her at the beginning, but now it is too late.
Note:
这里的ought to也可以用should代替。

比较：
Can / Could he have read the book? 他可能读过那本书吗？
He may / might have read the book. 他也许读过那本书。

He should（ought to ）have read the book. 他应该读过那本书。

He must have read the book. 他一定读过那本书。

He can't have read the book. 他不可能（决不会）读过那本书。

这两种结构表示：“应当正在…，可能正在…”，“应当一直在…，可能一直在…”等。

例如：
She might be still thinking of you.
她可能一直在想你。

He must have been working hard these years.
这些年来他一定是非常努力的。

比较→
对现在活动的推测：
What can / could she be doing now? 她现在会在做什么呢？She may / might be watering the flowers. 她可能在浇花。

She must be watering the flowers. 她一定在浇花。

She can't be watering the flowers.她不可能在浇花。

比较→
对一直进行的活动的推测：
He may have been writing the paper for months.
他也许几个月来一直在写论文。

He must have been writing the paper for months.
他肯定几个月来一直在写论文。

Can he have been writing the paper for months?
难道他几个月来一直在写论文吗？。