何学群
一概述
1. 英语中助动词主要有两类：一是基本助动词，如have, do, be；二是情态助动词。
2. 情态动词与其他动词连用表示说话人的语气。情态动词可表达建议、要求、可能和意愿等。
3. 情态动词没有人称和数的变化。
4. 情态动词的用法可分为4个大类：本义，规劝，推测，和责备（虚拟）用法
二 本义
A. 常用词：
1. may
1)表示请求、可以、允许; 否定答语要用must not,表示“不许可”、“不应该”、“不行”。
You may drive the tractor. 你可以开那台拖拉机。
May I come in? Yes, you may. No, you can’t (mustn’t, you’d better not.)
May I / we …？肯定回答: Yes, please/Certainly； Please don’t/No, you mustn’t.
“May we leave now?” “No, you mustn’t. You haven’t finished your home work yet.”
Might I trouble you for a light? “You may indeed.” “我可以向你借火吗？”“当然可以”
回答一般是 sure, certainly, of course,不能用might
2) 表祝愿 May you be happy! May you succeed！祝你成功！
2. must
1) 表示说话人的意志和义务。或要求对方必须作某事。
We must do everything step by step .我们必须按部就班地做一切事情。
You must be there on time. 你务必要按时到达那里。
2) 语气，“偏偏”
Why must you always bother me? 为什么你偏要打扰我呢。
3）must 的否定式：needn’t, don’t have to “不必”、“无须”、“用不着”、“不一定”。
must not “不应该”、“不许可”、“禁止”
--- Must I go tomorrow?明天我必须去吗？
--- Yes, please.是的, 请吧！No , you needn’t. 不，你不必去。
3 Shall
1）用于一、三人称疑问句、表征求对方意见或向对方请求。
-Shall he come in? - Come in, please. -可以让他进来吗？-请进。
What shall I wear on the journey? 我路上穿什么好呢？
Shall we dance? 我们跳舞好吗？
Shall he come to see you? 要不要他来看你？
2）用于二、三人称，表示保证、承诺、威胁、警告。
You shall be punished if you insist on doing it this way. 如果你坚持这样做就会受到惩罚。
If he passes the exam he shall have a holiday.
The law shall come into effect on May 1st.
注： 用于第一人称表将来，理解为助动词用法
I shall go to work after I have finished school. 毕业后，我会去工作。
4. should
1)表达说话人的感情如惊奇、愤怒、失望、竟然等，常用在why , how开头的疑问句中或that 引导的从句中
Why should you be so late? 为什么你竟这么晚？
How should I know?我怎么会知道？
I'm sorry that you should think so badly of me. 你竟把我想得那么坏，我感到遗憾。
You can’t imagine that a well-behaved gentleman should be so rude to a lady.
你不能想象行为端正的男士会对一个女士如此粗鲁。
I didn’t expect that he should have behaved like that. 我无法想象他竟然这样做。
I am sorry that she should be so careless. 我感到遗憾她竟会那样粗心。
2)一般不应被认为是情态动词shall的过去式，主要用法有：
A. 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，如：
What should we do now? 我们现在该怎么办？
B. 表示应该、必须，常与must 换用。例如：
We should (must) master a foreign language at least. 我们应当至少掌握一门外语。
It is suggested that he should go there by air. 建议他乘飞机去那里。
5. need
考试中主要测试 need 作情态动词与作实义动词的区别，need 作情态动词时，后面跟动词原形，即 “need do”。通常用在疑问句和否定句中，表示询问是否有“必要”，其否定形式为needn’t，表示“不必”；疑问形式为 Need … do? 极少用于肯定句。
1）I don’t think we need turn the light on at that time.
2）Need you ride a bike to school?
6. will
1) will作情态动词，表示意愿、习惯等。
It will be highly appreciated if you will send us your latest price list.
Boys will be boys. 男孩子究竟是男孩子。
No matter what you say, he will always argue with you. 不管你说什么，他总是和你争论。
A wise man changes his mind, a fool never will. 聪明人能随机应变，傻瓜是不会的。
2) 用于第二人称作主语的疑问句中，表示对对方的请求，would的语气比will委碗
Would/will you kindly tell me the way to the station? 请问到火车站怎么走？
Will/would you please do...,Would you like to do...,Would you mind doing...都表示建议、询问、请求
Will you please pass me a cup of tea? Would you like me to give you a hand?
This box is too heavy, ____give me a hand? (CET-4 1998,1)
A) would you mind B) would you please C) will you like to D) will you please to
3）表示意志，决心或愿望。例如：
Surely we will support all the people in the world in their struggle for peace.
我们一定要支持全世界人民争取和平的斗争。
He would not let me try it . 他不肯让我去试。
4）will表示经常性、习惯性、倾向性，would表示过去的习惯行为。
He will sit there hour after hour looking at the traffic go by.
他会经常一连几个小时坐在那儿观看来往的车辆。
Water will boil at 100℃. 水在摄氏100度时总是要沸腾的。
He would come to see me when he was in Beijing. 他在北京时，常来看望我。
On Sundays he would get up early and go fishing.
7. can (could) 表能力;
1) can表能力时意味着凭体力或脑力或技术等可以无甚阻力地去做某事。
I can climb this pole. 我能爬这根杆子。 Fire can’t destroy gold. 火烧不毁金子。
He is only four , but he can read. 他只有4岁，但已认得字了。
I could swim when I was only six. 我刚六岁就能游泳。
2) 允许（和may意思相近，委婉语气could）
Can (May) I come in ? 我能进来吗？Can I smoke here ? 我可以在这里抽烟吗？
Could I give you dinner one of these days? 这几天我请你吃顿饭行吗？
回答: yes, please; Yes, of course you can.
No, you mustn't /cannot/ may not 不用could
8. ought to
Ought to 后接动词原形，表义务，但不及must 那样具有信心，如：
You don’t look well. You ought to go to see the doctor. 你气色不好，应该去看病。
Ought to 用于否定句，其否定形式可缩略为oughtn’t ，如：
You oughtn’t to smoke so much. 你不应该抽这么多烟。
也可以用于疑问句： Ought you to smoke so much?你应该抽这样多烟吗？
Ought to 在间接引语中表过去时形式不变：
He said you ought to tell the police. 他说你应该去报告警察。
B. 要注意的几对词
1. can 和 be able to
1) can 与 be able to表示能力：can (could)表示主观能力，不表示意愿，
My grandma is over seventy, but she can still read without glasses.
He is able to give up his bad habits.
2) Can表示一贯的能力 ，be able to表示客观能力和通过努力可以达到的能力
I can’t swim. But I am sure I will be able to swim through more practicing.
The fire spread through the hotel, but everyone was able to get out
When the boat sank he was able to swim to the bank
3)表示将来式时，表具体做某一件事的能力时，则须用 be able to
You will be able to skate after you have practiced it two or three times.你练习两三次后就会溜冰了。
He was able to translate the article without a dictionary. 他可以不用词典翻译那篇文章。
2. must 和 have to “必须”
（1）must 强调“内在的职责”、“义务”，而have to 强调“外界压力”、“不得已而为之”。
You must come to the classroom before eight.
It is raining hard outside, but I have to leave now.
I must clean the room.（主观想法） I have to clean the room.（客观需要）
（2）have to多种时态，must一般用于现在时，其过去时had to, 将来时shall / will have to.
We had to be there at ten .我们得在十点钟到那里。
We will have to reconsider the whole thing. 这一切我们将不得不重新加以考虑。
（3） must否定回答，不用mustn’t，而用needn’t或don’t have to
“Must we do it now?” “No, you needn’t.”
3. Need 和dare： 考试中主要测试作情态动词和作实义动词的区别。
a. 有无助动词
b. 词尾有无s(need 除外，因为一般不用于陈述句)
⑴need 作情态动词，表示"需要"，只用于否定句和疑问句中，无人称和数的变化。
We needn't go there tomorrow. 我们不必明天去那里。
Need I tell you all the details? 我需要把一切详情告诉你吗？
⑵dare作情态动词，通常用在否定句和疑问句中，无人称和数的变化。
He dare not do so. 他不敢那么做。
I dare not stand on the chair. 我不敢站在椅子上。
How dare you say such a thing? 你怎么敢说这种话？
⑶need作行为动词，与一般动词的变化相同, 后面接"to + 动词原形"。
Do I need to tell you all the details? 我需要把一切详情告诉你吗？
It needs to be done at once. 这件事需要马上做。
You don't need to worry about that. 你不必为那事担心。
You didn't need to come. 你当时没必要来。
dare 也可作行为动词, 用在肯定句中（I dare say除外）和疑问句中，后面都接"to +动词原形"，在否定句中有时带to, 有时不带to, 如：
I dared to do so. 我敢于这样做。
She dares to go out alone at night. 她晚上敢一人出去。
Did he dare to do so? 他敢这样做吗？
He did not dare (to ) do so. 他不敢这样做。
⑷惯用语：I dare say 表示"我想"、"大概"。（dare say 也可以连写）
There's something wrong with the radio, I dare say. 收音机恐怕有点毛病。
I daresay you are right. 我想你是对的。
C. 用作情态动词的其他短语: would rather, would sooner, would (just) as soon, had rather, had better, had sooner, can not but, may (just) as well
1）The soldier would sooner die than surrender(投降).
2）The brave soldier would as soon die as yield（屈服）to such an enemy.
3）I’d rather walk than take a bus.
4）If you don’t like to swim, you may just as well stay at home.
注：这些短语后一般直接跟动词原形.
would (had) rather, would (had) sooner, would (just) as soon后可跟that从句，that 常省去，从句要用虚拟语气。对现在和将来的假设用过去时，对过去的假设用过去完成时。例如：
1）I would rather you came on Sunday.
2）I would sooner you hadn’t asked me to speak yesterday.
二 规劝
1. 肯定：may(might), may/might (just) as well ,should,
2. 否定：may not, cannot, must not
You can not smoke in the bus. 公共汽车上不许吸烟。
Can't you do it tomorrow? 你不能明天做吗？
Students may not stay out after midnight. 学生午夜后不得在外逗留。
You mustn't play football in the street, children. 孩子们，不准在马路上踢球。
3. 有些短语：
may/might (just) as well “不妨，最好”，与had better相近；
Since the flight was cancelled, you might as well go by train.
既然航班已经取消了，你不妨乘火车吧。相当于you had better go by train。
cannot / can’t…too …“越……越好，怎么也不过分”。变体：cannot…over…
You cannot be too careful when you drive a car.驾车时候，越小心越好。
The final chapter covers organizational change and development. This subject cannot be over emphasized.
三 推测：
一）．搞清楚两条线索：
1．意义上分为肯定的推测和否定的推测两种；
must , will, should, may, might , could表示可能性依次减小；
can’t, couldn’t, may not, might not不可能性依次减小
“I wonder how Tom knew about your past.” “He must / could / may / might have heard of it from Mary.”
He must tell a lie. 他肯定是说谎。Can he tell a lie? 他会说谎吗？He can’t tell a lie. 他不会说谎的。
Since the ditch is full of water, it must have rained last night.
You couldn’t have met my grandmother. She died before you were born.
Could /Can it have happened last night ? 它在昨晚发生了吗？
It may rain tomorrow. 明天可能下雨。 (I'm told by the radio.)
What you said might be true. 你说的可能对。
She may be staying at home.
Anybody can make mistake.任何人都可能犯错误。
The weather here could be very cold in winter. 冬天这的天气有时很冷。
2 时间上分为现在，将来和过去
A) 现在：(一般情况和强调推测正在进行) (must be; must do; must be doing)
Can it be true? 这会是真的吗？
He must be playing basketball in the room.
B）过去：（一般情况和强调推测过去正在进行）(must have done; must have been doing)
“The dictionary has disappeared. Who could have taken it?”
“Tom could have taken it. He was alone yesterday.”
I can’t find my keys. I may / might have left them at the school yesterday.
What you referred to just now can have made her very sad. 你刚刚所谈到的可能令他很伤心。
He might not have settled the question. 他可能尚未解决那个问题。
She must have studied English before.她以前一定学过英语。
They should have been meeting to discuss the problem.
He may / might have been buying stamps in the post office when you saw him.
They are sweating all over. They must have been working in the fields.
They may have been discussing the problem this morning.
She couldn’t have been swimming all day.
Can they have won the basketball match？他们赢了那场篮球赛吗？
C）将来：参见下面具体的情态动词 (形式同现在)
注意：适当考虑时态变化 could, might等大多应认为是语气较弱。
Electric irons could be dangerous; they might give you a severe shock. 电熨斗会有危险, 可能电着人。
二） 其他具体的可能性：
1． can多用于否定与疑问结构中，但也可用在肯定句中,表示潜在的或理论上的可能性
Can the news be true? 这消息可能是真的吗？
It can’t be true. 它不可能是真的。
What can he possibly mean? 他可能是什么意思？
A house in the center of London can cost a lot of money.
Attending the ball can be very exciting.
The road can be blocked. 这条路可能会不通的。
may, might表示事实上的可能性。
The road may be blocked. 这条路可能不通了。
He may have done the work last night, didn’t he?
She may be staying at home.
对比：
The railways may be improved. =It is possible that the railways will be improved.
=Perhaps/Maybe/Possibly the railways will be improved.
铁路可能会得到改进。（意味着已有具体的改进计划或方案。）
The railways can be improved.=It is possible for the railways to be improved.
铁路可以得到改进。（意思是铁路还不完善，尚有问题存在。）
He looks pale. He may be ill.他脸色苍白，可能有病。
He is in poor health. He can be ill at any time.他体质很差，任何时候都有可能生病。
2．will表示说话人对将来和现在的预测，表示某种倾向和习惯性,尤其指在某个条件下。
I think he will be all right now. 我想他现在一定好了。
That would be his mother. 那肯定是他母亲。
Oil will float on water. 油浮在水面上。
He'll talk for hours if you give the chance. 如果给他机会，他会谈上几个小时。
3. must只用在肯定句中
He must be an honest boy. 他一定是个诚实的男孩。
He must have finished the work, hasn’t he?
He must be playing basketball in the room.
4. should (ought to)
表示很可能、预期的意思，指一种尝试性推论，是must的语气较弱的对应词，”应该会、一定---吧”。比较：Our guests must be home by now.(‘I am certain’)我们的客人现在一定到家了。
（根据他们动身的时间、路程远近、速度等具体条件，我能断定/肯定他们现在已经到家了。）
Our guests should(ought to) be home by now.(‘They probably are, but I’m not certain.)
我们的客人现在该到家了。（含义是我想他们很可能到家了，但我不能肯定”）
It's nearly seven o'clock.. Jack ____be here at any moment. (NMET95)
A .must B. need C. should D. can
三）表猜测语气的反意疑问句的构成
He must be reading, isn’t he? 他肯定在读书，不是吗？
He must/may be in the room, isn’t he?
He can’t be in the room, is he？
You must do it yourself, don’t you? 你一定要自己做，不是吗？
He must have gone over the article, hasn’t he? 他一定已经检查了这篇论文，不是吗？
It must have rained last night, didn’t it? 昨晚一定下过雨了，不是吗？
Mary ______my letter; otherwise she would have replied before now.
[A] couldn’t have received [B] ought to have received
[C] has received [D] shouldn’t have received
四 责备: (虚拟语气)
1. 情态动词的选用考虑该词的本义
2．同样需要分清时间
A）现在：
You ought not to make fun of him. He is not the one you laugh at but learn from.
在“It is natural (strange, natural, necessary, surprised, impossible, important ) that……”句型中，
在lest（以免）、for fear (that) (以防)、in case（以备万一）等之后也要用should+动词原形;
在advise, sugest, order, demand, request 等的从句中;
谓语动词要用should +动词原形, 表示“理所当然”、“奇怪”、“必要”、“惊异”等的意思。
It is necessary that he(should) be sent there at once. 有必要马上派他到那里去。
It is strange that he should say so. 他会说这样的话真是奇怪。
Let us go at once lest we should be late for the train. 我们马上走吧，以免赶不上火车。
B）过去：（情态动词+have done/been）
You should have come here five minutes earlier. 你应该早来5分钟的。
With all the work finished, I should have gone to the party last night.
You needn’t have waken me up; I don’t have to go to work today.
You needn’t have come over yourself.
I regret having left the work unfinished; I should have planned everything ahead carefully.
我本来应该事先认真地把每件事情规划的很好，但实际上作者没有规划好，以至工作没有完成。
What you said is right, but you could have phrased it more tactfully.
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）
You could have completed the task a little earlier. 你本来能早点完成任务的。
I could have passed my examination easily but I made too many stupid mistakes.
我本可以轻易通过考试，但我犯了太多不该犯的错误。
例题：
As it turned out to be a small house party, we ______so formally.
[A] needn’t dress up [B]did not need have dressed up
[C] did not need dress up [D] needn’t have dressed up
C) 将来：常用在if 从句中，构成虚拟语气。
If it should/were to------, ____________________________.
If he should drop in, give him my message. 他如果来访，就把我的条交给他。
= Should he drop in, give him my message.
Ask her to ring me up if you should see her. 你如若见到她，让她给我打个电话。
If it should snow tomorrow , the meeting would be put off．如果明天下雪的话，运动会就会推迟。
Should it blow hard，we would not go out for a picnic．万一明天刮大风，我们就无法去野餐了。
五 注意点
1．当试题的前句和后句在动作和意义上相互补充说明，且整个句意在动作和时间上是一个整体时，我们可用“统一”关系来解决这样的试题。常见的结构有：
must have done： 表示对过去动作的肯定推测，常译作“一定做了……”，只能用于肯定句中。
can’t／couldn’t have done牱穸ㄐ问
Can／Could...have done﹖ 疑问式
could ／might have done：表示对过去发生的动作的可能性推测，常译作“可能做了……”。
例题：
1) Sorry I’m late．I ____have turned off the alarm clock and gone back to sleep again(北京 2000春)
A． might B． should C． can D． will
该题前句说明了结果，后句接着说出了产生这种结果的可能性，对前句进行补充说明。
2) My sister met him at the Grand Theater yesterday afternoon, so he ____ your lecture．(上海 2000)
A. couldn’t have attended B. needn’t have attended
C. mustn’t have attended D. shouldn’t have attended
该题前句叙说一个客观事实，后句对前句进行补充说明，分析选项可知C是错误的
3) Jack ＿＿＿＿ yet, otherwise he would have telephoned me． (上海’97)
A. mustn’t have arrived B. shouldn’t have arrived
C. can’t have arrived D. need not have arrived
2．当试题的前后句在动作和意义上构成转折关系时，常借助“but, however, instead”等词来表示过去的动作与客观事实不符，这时我们就可以用“对立”关系来解决这样的试题。常见的有：
should have done /ought to have done：表示过去本应该做某事而实际上没有做。
should not have done /ought not to have done：表示过去本不应该做某事但事实上却做了。
need have done：表示过去本来有必要去做某事，但事实上没有做。
need not have done：表示过去本来没有必要做某事，但事实上却做了。
4) I was really anxious about you． You ______ home without a word． (NMET2001)
A. mustn’t leave B. shouldn’t have left C. couldn’t have left D. needn’t leave
分析该题前后句之间的关系和语气可知，事实上是“本不应该离家出走却走了”
5) I told Sally how to get here, but perhaps I ______ for her． (NMET’94)
A. had to write it out B. must have written it out
C. should have written it out D. ought to write it out
由句中的连词but可知前后句之间是对立关系
3. 容易混淆的形式：
1）should have done: 推测和责备
---I sent him a letter last Monday.
---Really? So I guess he _____ it by now.
A. should receive B. should have received C. will receive D. could receive
六 总结
一、考查情态动词的基本用法
一些常见情态动词的非常用意义往往为高考考查的焦点，考生须确切理解其含义并根据句子意思选择正确的情态动词。
例1：“The interest ______be divided into five parts, according to the agreement made by both sides.” declared the judge. (2004重庆)
A. may B. should C. must D. shall
例2：- The room is so dirty. ______ we clean it?
- Of course. (2003北京春)
A. Will B. Shall C. Would D. Do
分析：上述两题均考查 shall的非常用含义，当shall用于一、三人称疑问句中时，表示征求对方意见；用于二、三人称陈述句中，表示说话人给对方的命令、警告、允诺或威胁等。
例3：He’s strange -- he ____ sit for hours without saying anything.
A. shall B. will C. can D. must
分析：will用来谈习惯和特性，尤其是不受时限必然发生的情况。本句意思为“他很怪，能一坐几个小时一句话也不说”。
例4：John, look at the time. _______ you play the piano at such a late hour？(2005全国Ⅲ)
A. Must B. Can C. May D. Need
例5：Tom, you _____leave all your clothes on the floor like this！(2005全国II)
A. wouldn’t B. mustn’t C. needn’t D. may not
分析：must用于疑问句，表示责备、抱怨的感情色彩，意思为“偏要，硬要、干嘛”；而mustn’t则表示禁止、不允许，是说话人强有力的劝告。
例6：You can’t imagine that a well-behaved gentleman _____ be so rude to a lady. (2001上海)
A. might B. need C. should D. would
分析：在某些名词性从句中，should 表示惊异、意外等情绪，常译为“竟然”。
如：It seems unfair that this should happen to me. 真不公平，这件事竟然发生在我身上。
例7：_____it rain tomorrow, the meeting would be put off.
A. Should B. Would C. Could D. Must
分析：在条件状语从句中，should 表示“万一”，即可表示不能实现的假设，又可表示可以实现的假设。该句意思为“万一明天下雨，会议就延期。”
例8 -Is John coming by train﹖
-He should, but he ＿＿＿＿＿＿ not． He likes driving his car． (NMET2002)
A. must B. can C. need D. may
分析: mustn’t 表示“禁止、不准”；cannot 表示“不可能”；need not 表示“不必要”；may not 表示“可能不”。
例9 -I hear you’ve got a set of valuable Australian coins．________ I have a look﹖
-Yes, certainly． (北京2002春) A． Do B． May C． Shall D．Should
分析: 这是在征求对方的许可，may表示“允许、可以”，语气比较委婉 shall常用于第一、三人称作主语的疑问句中,表示征求对方意见和指示，如果此空用shall，则意为“要（我）看一下吗？”不符合上下文意思。
例10. I should have been there, but I ______ not find the time． (上海2000春)
A． would B．could C． might D． should
分析: 第二个分句表示过去的某种能力；C 项只表示语气上的可能性，与题意不符。
例11. Johnny, you ______ play with the knife, you ______hurt yourself． (NMET’96)
A. won’t; can’t B. mustn’t; may C. shouldn’t; must D. can’t; shouldn’t
分析: mustn’t 表示“不可以；禁止”，分析题意可知第二个空表示某种可能性
例12 -Will you stay for lunch﹖
-Sorry, ______． My brother is coming to see me． (NMET’99)
A． I mustn’t B． I can’t C． I needn’t D． I won’t
分析: 因为“我弟弟要来看我”，所以“不能留下”，因此对别人的邀请或要求应给予礼貌的拒绝。A 项表示“禁止”；C项表示“不必要”；而D项表示“不会”，均不符合题意。
例13-Could I borrow your dictionary﹖
-Yes, of course you ＿＿＿＿＿．(MET’92)
A． might B． will C． can D. should
例14 --- When can I come for the photos﹖ I need them tomorrow afternoon．
--- They ______ be ready by 12:00． (NMET’98)
A. can B. should C. might D. need
例15 The fire spread through the hotel very quickly but everyone ______ get out．(NMET’97)
A. had to B. would C. could D. was able to
分析: 该题考查了could和be able to的区别，二者都可表示过去时间的能力，但如果表示过去成功地做了某事只能使用was /were able to do
例16 -Shall I tell John about it ﹖
-No, you _______． I’ve told him already． (NMET’94)
A. needn’t B. wouldn’t C. mustn’t D. shouldn’t
情态动词shall在试题中表示征询对方意见或请求指示。答句暗示 “没有必要了”
二、考查情态动词的推测用法
1、对现在或客观事实的推测
对现在或客观事实的肯定推测一般用must, will, should, may(might)或could，语气依次递减。
例17：Helen _____go on the trip with us but she isn’t quite sure yet. (2005安徽)
A. shall B. must C. may D. can
例18：-I’ve taken someone else’s green sweater by mistake.
-It ___ Harry’s. He always wears green. (2005广东)
A. has to be B. will be C. mustn’t be D. could be
例19：That, I think, ____ be all that I want to say.
A. can B. could C. will D. ought to
分析： must的语气最强，译为“肯定”、“准是”、“想必是”；should的语气次之，译为“很可能”、“应该”，指按常理推测；may(might)译为“也许、大概”，表示把握不大的推断，后面常跟but。could的语气最弱，译为“也许”、“可能”。
对现在或客观事实的否定推测，常用cannot译为“根本不可能”, 表示推测的语气非常肯定, may not, might not或could not译为“可能不”、“也许不”，表示推测的语气不很肯定。
例20：A left-luggage office is a place where bags ____ be left for a short time, especially at a railway station.
A. should B. can C. must D. will (2003全国卷)
例21：The World Wide Web is sometimes jokingly called the World Wide Wait because it ___be very slow.
A. should B. must C. will D. can (2005浙江)
分析： can 表示推测时常用于疑问和否定句中，用于肯定句中常常表示理论上的可能性，用来表达事件或情况可能发生(但并不牵涉到是否真的会发生)。
例22：You might just as well tell the manufacturer that male customers __not like the design of the furniture.
A. must B. shall C. may D. need (2004上海春)
例23：--Isn’t that Ann’s husband over there?
--No, it _______ be him --I’m sure he doesn’t wear glasses. (2004全国Ⅰ)
A. can’t B. must not C. won’t D. may not
分析：might just as well表示“最好……，还是……为好”，说明喜欢的可能性不大；根据后面的I’m sure可以看出，推测的语气较强，故用can’t译为“根本不可能”、“想必不会”，表示惊异、怀疑的感情色彩，常用于否定语气较强时。
例24：Mr. Bush is on time for everything. How ______ it be that he was late for the opening ceremony？
A. can B. should C. may D. must (2001上海春)
分析：对现在或客观事实的疑问推测, 用can或could，表惊异、怀疑、困惑、失望等感情色彩。
2、对过去事实的推测
对过去已发生事情的肯定推测常用 “must/may/might +have + v-ed”，“must+ have + v-ed”意思为“想必、准是、一定是”，语气非常肯定且只能用于肯定句，“may/might+ have + v-ed” 意思为“也许、或许”表示推测的把握不大，只能用于肯定或否定句，不用于疑问句；对过去的否定推测常用 “can/could /may’might not + have + v-ed”，意思为“不可能/可能没做过……”；对过去的疑问推测只用“can/could + have + v-ed”，意思为“可能已经……”。
例25：He _______ have completed his work; otherwise, he wouldn’t be enjoying himself by seaside.
A. should B. must C. wouldn’t D. can’t (2005北京)
分析：根据后一分句判断，前一分句应该表示肯定完成了工作，故用“must+ have + v-ed”意思为“想必、准是、一定是”，语气非常肯定且只能用于肯定句。
例26：My sister met him at the Grand Theatre yesterday afternoon, so he _____ your lecture.(2000上海)
A. couldn’t have attended B. needn’t have attended
C. mustn’t have attended D. shouldn’t have attended
分析：must表推测时，不用于否定，needn’t have +v-ed 表示“本来没有必要”，shouldn’t have +v-ed表示“本来不应该”，“can/could not + have + v-ed” 意思为“不可能做过……”。
例27：--Do you know where David is? I couldn’t find him anywhere.
-- Well. He ______ have gone far----his coat’s still here.(2005湖北)
A. shouldn’t B. mustn’t C. can’t D. wouldn’t
分析：can’t have gone far 在此表示现在对过去所发生事情的否定推测，“现在不可能走远了”
3、对将来事实的推测
例28 -Are you coming to Jeff’s party﹖
-I’m not sure． I ________ go to the concert instead． (NMET2000)
A． must B．would C． should D． might
分析：由题意和下句中的 “I’m not sure” 可知这段对话中存在一种可能性推测，might可以用来表示一种比较委婉的可能性判断
三、考查情态动词的虚拟用法
例29 ---I’ll tell Mary about her new job tomorrow.
---You________ her last week. (2004福建)
A. ought to tell B. would have told C. must tell D. should have told
分析：对过去发生事情的责备或遗憾应用“情态动词+have + v-ed”，据此可排除答案A、C，而would +have +v-ed常用于过去的虚拟语气中，should(ought to)+ have + v-ed”表示本应该做某事而实际上没有做，含有责备的含义。其否定式表示某种行为不该发生但却发生了。故答案为D。
例30：He paid for a seat, when he ______ have entered free. (2005山东)
A. could B. would C. must D. need
分析：“could+ have + v-ed” 用于肯定句中，表示本来能够做成某事的但结果没能做成，含有遗憾的意味，侧重有能力而没能做成。“他花钱买了座位，而他本来可以免费进去的”。
例31：You should not have swum in that river. You ____ have been eaten by fish.
A. must B. may C. will D. might
分析：前一分句的should not have swum表示责备，后一分句则表明有被鱼吃掉的可能性。“might+ have + v-ed” 用于肯定句中，表示本来有可能做成某事但结果没能做成，含有遗憾的意味，侧重有某种可能性但没能做成。
例32：-Catherine, I have cleaned the room for you.
-Thanks. You ______ it. I could manage it myself. (2005福建)
A. needn’t do B. needn’t have done C. mustn’t do D. shouldn’t have done
分析：“needn’t+ have + v-ed” 表示本来不必做某事而实际上做了某事，无批评责备之意，只是惋惜而已。如You needn’t have watered the flowers, for it is going to rain.你本不需要浇花的，因为天就要下雨了。