何學群
一概述
1. 英語中助動詞主要有兩類：一是基本助動詞，如have, do, be；二是情態助動詞。
2. 情態動詞與其他動詞連用表示說話人的語氣。情態動詞可表達建議、要求、可能和意愿等。
3. 情態動詞沒有人稱和數的變化。
4. 情態動詞的用法可分為4個大類：本義，規勸，推測，和責備（虛擬）用法
二 本義
A. 常用詞：
1. may
1)表示請求、可以、允許; 否定答語要用must not,表示“不許可”、“不應該”、“不行”。
You may drive the tractor. 你可以開那臺拖拉機。
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.” “我可以向你借火嗎？”“當然可以”
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) 表示說話人的意志和義務。或要求對方必須作某事。
We must do everything step by step .我們必須按部就班地做一切事情。
You must be there on time. 你務必要按時到達那里。
2) 語氣，“偏偏”
Why must you always bother me? 為什么你偏要打擾我呢。
3）must 的否定式：needn’t, don’t have to “不必”、“無須”、“用不著”、“不一定”。
must not “不應該”、“不許可”、“禁止”
--- Must I go tomorrow?明天我必須去嗎？
--- Yes, please.是的, 請吧！No , you needn’t. 不，你不必去。
3 Shall
1）用于一、三人稱疑問句、表征求對方意見或向對方請求。
-Shall he come in? - Come in, please. -可以讓他進來嗎？-請進。
What shall I wear on the journey? 我路上穿什么好呢？
Shall we dance? 我們跳舞好嗎？
Shall he come to see you? 要不要他來看你？
2）用于二、三人稱，表示保證、承諾、威脅、警告。
You shall be punished if you insist on doing it this way. 如果你堅持這樣做就會受到懲罰。
If he passes the exam he shall have a holiday.
The law shall come into effect on May 1st.
注： 用于第一人稱表將來，理解為助動詞用法
I shall go to work after I have finished school. 畢業后，我會去工作。
4. should
1)表達說話人的感情如驚奇、憤怒、失望、竟然等，常用在why , how開頭的疑問句中或that 引導的從句中
Why should you be so late? 為什么你竟這么晚？
How should I know?我怎么會知道？
I'm sorry that you should think so badly of me. 你竟把我想得那么壞，我感到遺憾。
You can’t imagine that a well-behaved gentleman should be so rude to a lady.
你不能想象行為端正的男士會對一個女士如此粗魯。
I didn’t expect that he should have behaved like that. 我無法想象他竟然這樣做。
I am sorry that she should be so careless. 我感到遺憾她竟會那樣粗心。
2)一般不應被認為是情態動詞shall的過去式，主要用法有：
A. 用于第一人稱疑問句中詢問對方的意愿，但語氣較委婉溫和，如：
What should we do now? 我們現在該怎么辦？
B. 表示應該、必須，常與must 換用。例如：
We should (must) master a foreign language at least. 我們應當至少掌握一門外語。
It is suggested that he should go there by air. 建議他乘飛機去那里。
5. need
考試中主要測試 need 作情態動詞與作實義動詞的區別，need 作情態動詞時，后面跟動詞原形，即 “need do”。通常用在疑問句和否定句中，表示詢問是否有“必要”，其否定形式為needn’t，表示“不必”；疑問形式為 Need … do? 極少用于肯定句。
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作情態動詞，表示意愿、習慣等。
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. 不管你說什么，他總是和你爭論。
A wise man changes his mind, a fool never will. 聰明人能隨機應變，傻瓜是不會的。
2) 用于第二人稱作主語的疑問句中，表示對對方的請求，would的語氣比will委碗
Would/will you kindly tell me the way to the station? 請問到火車站怎么走？
Will/would you please do...,Would you like to do...,Would you mind doing...都表示建議、詢問、請求
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，決心或愿望。例如：
Surely we will support all the people in the world in their struggle for peace.
我們一定要支持全世界人民爭取和平的斗爭。
He would not let me try it . 他不肯讓我去試。
4）will表示經常性、習慣性、傾向性，would表示過去的習慣行為。
He will sit there hour after hour looking at the traffic go by.
他會經常一連幾個小時坐在那兒觀看來往的車輛。
Water will boil at 100℃. 水在攝氏100度時總是要沸騰的。
He would come to see me when he was in Beijing. 他在北京時，常來看望我。
On Sundays he would get up early and go fishing.
7. can (could) 表能力;
1) can表能力時意味著憑體力或腦力或技術等可以無甚阻力地去做某事。
I can climb this pole. 我能爬這根桿子。 Fire can’t destroy gold. 火燒不毀金子。
He is only four , but he can read. 他只有4歲，但已認得字了。
I could swim when I was only six. 我剛六歲就能游泳。
2) 允許（和may意思相近，委婉語氣could）
Can (May) I come in ? 我能進來嗎？Can I smoke here ? 我可以在這里抽煙嗎？
Could I give you dinner one of these days? 這幾天我請你吃頓飯行嗎？
回答: yes, please; Yes, of course you can.
No, you mustn't /cannot/ may not 不用could
8. ought to
Ought to 后接動詞原形，表義務，但不及must 那樣具有信心，如：
You don’t look well. You ought to go to see the doctor. 你氣色不好，應該去看病。
Ought to 用于否定句，其否定形式可縮略為oughtn’t ，如：
You oughtn’t to smoke so much. 你不應該抽這么多煙。
也可以用于疑問句： Ought you to smoke so much?你應該抽這樣多煙嗎？
Ought to 在間接引語中表過去時形式不變：
He said you ought to tell the police. 他說你應該去報告警察。
B. 要注意的幾對詞
1. can 和 be able to
1) can 與 be able to表示能力：can (could)表示主觀能力，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表示一貫的能力 ，be able to表示客觀能力和通過努力可以達到的能力
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)表示將來式時，表具體做某一件事的能力時，則須用 be able to
You will be able to skate after you have practiced it two or three times.你練習兩三次后就會溜冰了。
He was able to translate the article without a dictionary. 他可以不用詞典翻譯那篇文章。
2. must 和 have to “必須”
（1）must 強調“內在的職責”、“義務”，而have to 強調“外界壓力”、“不得已而為之”。
You must come to the classroom before eight.
It is raining hard outside, but I have to leave now.
I must clean the room.（主觀想法） I have to clean the room.（客觀需要）
（2）have to多種時態，must一般用于現在時，其過去時had to, 將來時shall / will have to.
We had to be there at ten .我們得在十點鐘到那里。
We will have to reconsider the whole thing. 這一切我們將不得不重新加以考慮。
（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： 考試中主要測試作情態動詞和作實義動詞的區別。
a. 有無助動詞
b. 詞尾有無s(need 除外，因為一般不用于陳述句)
⑴need 作情態動詞，表示"需要"，只用于否定句和疑問句中，無人稱和數的變化。
We needn't go there tomorrow. 我們不必明天去那里。
Need I tell you all the details? 我需要把一切詳情告訴你嗎？
⑵dare作情態動詞，通常用在否定句和疑問句中，無人稱和數的變化。
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? 你怎么敢說這種話？
⑶need作行為動詞，與一般動詞的變化相同, 后面接"to + 動詞原形"。
Do I need to tell you all the details? 我需要把一切詳情告訴你嗎？
It needs to be done at once. 這件事需要馬上做。
You don't need to worry about that. 你不必為那事擔心。
You didn't need to come. 你當時沒必要來。
dare 也可作行為動詞, 用在肯定句中（I dare say除外）和疑問句中，后面都接"to +動詞原形"，在否定句中有時帶to, 有時不帶to, 如：
I dared to do so. 我敢于這樣做。
She dares to go out alone at night. 她晚上敢一人出去。
Did he dare to do so? 他敢這樣做嗎？
He did not dare (to ) do so. 他不敢這樣做。
⑷慣用語：I dare say 表示"我想"、"大概"。（dare say 也可以連寫）
There's something wrong with the radio, I dare say. 收音機恐怕有點毛病。
I daresay you are right. 我想你是對的。
C. 用作情態動詞的其他短語: 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：這些短語后一般直接跟動詞原形.
would (had) rather, would (had) sooner, would (just) as soon后可跟that從句，that 常省去，從句要用虛擬語氣。對現在和將來的假設用過去時，對過去的假設用過去完成時。例如：
1）I would rather you came on Sunday.
2）I would sooner you hadn’t asked me to speak yesterday.
二 規勸
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. 公共汽車上不許吸煙。
Can't you do it tomorrow? 你不能明天做嗎？
Students may not stay out after midnight. 學生午夜后不得在外逗留。
You mustn't play football in the street, children. 孩子們，不準在馬路上踢球。
3. 有些短語：
may/might (just) as well “不妨，最好”，與had better相近；
Since the flight was cancelled, you might as well go by train.
既然航班已經取消了，你不妨乘火車吧。相當于you had better go by train。
cannot / can’t…too …“越……越好，怎么也不過分”。變體：cannot…over…
You cannot be too careful when you drive a car.駕車時候，越小心越好。
The final chapter covers organizational change and development. This subject cannot be over emphasized.
三 推測：
一）．搞清楚兩條線索：
1．意義上分為肯定的推測和否定的推測兩種；
must , will, should, may, might , could表示可能性依次減小；
can’t, couldn’t, may not, might not不可能性依次減小
“I wonder how Tom knew about your past.” “He must / could / may / might have heard of it from Mary.”
He must tell a lie. 他肯定是說謊。Can he tell a lie? 他會說謊嗎？He can’t tell a lie. 他不會說謊的。
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 ? 它在昨晚發生了嗎？
It may rain tomorrow. 明天可能下雨。 (I'm told by the radio.)
What you said might be true. 你說的可能對。
She may be staying at home.
Anybody can make mistake.任何人都可能犯錯誤。
The weather here could be very cold in winter. 冬天這的天氣有時很冷。
2 時間上分為現在，將來和過去
A) 現在：(一般情況和強調推測正在進行) (must be; must do; must be doing)
Can it be true? 這會是真的嗎？
He must be playing basketball in the room.
B）過去：（一般情況和強調推測過去正在進行）(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. 你剛剛所談到的可能令他很傷心。
He might not have settled the question. 他可能尚未解決那個問題。
She must have studied English before.她以前一定學過英語。
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？他們贏了那場籃球賽嗎？
C）將來：參見下面具體的情態動詞 (形式同現在)
注意：適當考慮時態變化 could, might等大多應認為是語氣較弱。
Electric irons could be dangerous; they might give you a severe shock. 電熨斗會有危險, 可能電著人。
二） 其他具體的可能性：
1． can多用于否定與疑問結構中，但也可用在肯定句中,表示潛在的或理論上的可能性
Can the news be true? 這消息可能是真的嗎？
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. 這條路可能會不通的。
may, might表示事實上的可能性。
The road may be blocked. 這條路可能不通了。
He may have done the work last night, didn’t he?
She may be staying at home.
對比：
The railways may be improved. =It is possible that the railways will be improved.
=Perhaps/Maybe/Possibly the railways will be improved.
鐵路可能會得到改進。（意味著已有具體的改進計劃或方案。）
The railways can be improved.=It is possible for the railways to be improved.
鐵路可以得到改進。（意思是鐵路還不完善，尚有問題存在。）
He looks pale. He may be ill.他臉色蒼白，可能有病。
He is in poor health. He can be ill at any time.他體質很差，任何時候都有可能生病。
2．will表示說話人對將來和現在的預測，表示某種傾向和習慣性,尤其指在某個條件下。
I think he will be all right now. 我想他現在一定好了。
That would be his mother. 那肯定是他母親。
Oil will float on water. 油浮在水面上。
He'll talk for hours if you give the chance. 如果給他機會，他會談上幾個小時。
3. must只用在肯定句中
He must be an honest boy. 他一定是個誠實的男孩。
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、預期的意思，指一種嘗試性推論，是must的語氣較弱的對應詞，”應該會、一定---吧”。比較：Our guests must be home by now.(‘I am certain’)我們的客人現在一定到家了。
（根據他們動身的時間、路程遠近、速度等具體條件，我能斷定/肯定他們現在已經到家了。）
Our guests should(ought to) be home by now.(‘They probably are, but I’m not certain.)
我們的客人現在該到家了。（含義是我想他們很可能到家了，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）表猜測語氣的反意疑問句的構成
He must be reading, isn’t he? 他肯定在讀書，不是嗎？
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，不是嗎？
He must have gone over the article, hasn’t he? 他一定已經檢查了這篇論文，不是嗎？
It must have rained last night, didn’t it? 昨晚一定下過雨了，不是嗎？
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 責備: (虛擬語氣)
1. 情態動詞的選用考慮該詞的本義
2．同樣需要分清時間
A）現在：
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（以備萬一）等之后也要用should+動詞原形;
在advise, sugest, order, demand, request 等的從句中;
謂語動詞要用should +動詞原形, 表示“理所當然”、“奇怪”、“必要”、“驚異”等的意思。
It is necessary that he(should) be sent there at once. 有必要馬上派他到那里去。
It is strange that he should say so. 他會說這樣的話真是奇怪。
Let us go at once lest we should be late for the train. 我們馬上走吧，以免趕不上火車。
B）過去：（情態動詞+have done/been）
You should have come here five minutes earlier. 你應該早來5分鐘的。
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.
我本來應該事先認真地把每件事情規劃的很好，但實際上作者沒有規劃好，以至工作沒有完成。
What you said is right, but you could have phrased it more tactfully.
I should have thought of that. 這一點我是應當想到的。（但沒想到）
They should not have left so soon.他們不應當走得這么早。（但已走了）
You could have completed the task a little earlier. 你本來能早點完成任務的。
I could have passed my examination easily but I made too many stupid mistakes.
我本可以輕易通過考試，但我犯了太多不該犯的錯誤。
例題：
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) 將來：常用在if 從句中，構成虛擬語氣。
If it should/were to------, ____________________________.
If he should drop in, give him my message. 他如果來訪，就把我的條交給他。
= Should he drop in, give him my message.
Ask her to ring me up if you should see her. 你如若見到她，讓她給我打個電話。
If it should snow tomorrow , the meeting would be put off．如果明天下雪的話，運動會就會推遲。
Should it blow hard，we would not go out for a picnic．萬一明天刮大風，我們就無法去野餐了。
五 注意點
1．當試題的前句和后句在動作和意義上相互補充說明，且整個句意在動作和時間上是一個整體時，我們可用“統一”關系來解決這樣的試題。常見的結構有：
must have done： 表示對過去動作的肯定推測，常譯作“一定做了……”，只能用于肯定句中。
can’t／couldn’t have done牱穸ㄐ問
Can／Could...have done﹖ 疑問式
could ／might have done：表示對過去發生的動作的可能性推測，常譯作“可能做了……”。
例題：
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
該題前句說明了結果，后句接著說出了產生這種結果的可能性，對前句進行補充說明。
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
該題前句敘說一個客觀事實，后句對前句進行補充說明，分析選項可知C是錯誤的
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．當試題的前后句在動作和意義上構成轉折關系時，常借助“but, however, instead”等詞來表示過去的動作與客觀事實不符，這時我們就可以用“對立”關系來解決這樣的試題。常見的有：
should have done /ought to have done：表示過去本應該做某事而實際上沒有做。
should not have done /ought not to have done：表示過去本不應該做某事但事實上卻做了。
need have done：表示過去本來有必要去做某事，但事實上沒有做。
need not have done：表示過去本來沒有必要做某事，但事實上卻做了。
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
分析該題前后句之間的關系和語氣可知，事實上是“本不應該離家出走卻走了”
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
由句中的連詞but可知前后句之間是對立關系
3. 容易混淆的形式：
1）should have done: 推測和責備
---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 總結
一、考查情態動詞的基本用法
一些常見情態動詞的非常用意義往往為高考考查的焦點，考生須確切理解其含義并根據句子意思選擇正確的情態動詞。
例1：“The interest ______be divided into five parts, according to the agreement made by both sides.” declared the judge. (2004重慶)
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：上述兩題均考查 shall的非常用含義，當shall用于一、三人稱疑問句中時，表示征求對方意見；用于二、三人稱陳述句中，表示說話人給對方的命令、警告、允諾或威脅等。
例3：He’s strange -- he ____ sit for hours without saying anything.
A. shall B. will C. can D. must
分析：will用來談習慣和特性，尤其是不受時限必然發生的情況。本句意思為“他很怪，能一坐幾個小時一句話也不說”。
例4：John, look at the time. _______ you play the piano at such a late hour？(2005全國Ⅲ)
A. Must B. Can C. May D. Need
例5：Tom, you _____leave all your clothes on the floor like this！(2005全國II)
A. wouldn’t B. mustn’t C. needn’t D. may not
分析：must用于疑問句，表示責備、抱怨的感情色彩，意思為“偏要，硬要、干嘛”；而mustn’t則表示禁止、不允許，是說話人強有力的勸告。
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：在某些名詞性從句中，should 表示驚異、意外等情緒，常譯為“竟然”。
如：It seems unfair that this should happen to me. 真不公平，這件事竟然發生在我身上。
例7：_____it rain tomorrow, the meeting would be put off.
A. Should B. Would C. Could D. Must
分析：在條件狀語從句中，should 表示“萬一”，即可表示不能實現的假設，又可表示可以實現的假設。該句意思為“萬一明天下雨，會議就延期。”
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、不準”；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: 這是在征求對方的許可，may表示“允許、可以”，語氣比較委婉 shall常用于第一、三人稱作主語的疑問句中,表示征求對方意見和指示，如果此空用shall，則意為“要（我）看一下嗎？”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: 第二個分句表示過去的某種能力；C 項只表示語氣上的可能性，與題意不符。
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”，分析題意可知第二個空表示某種可能性
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: 因為“我弟弟要來看我”，所以“不能留下”，因此對別人的邀請或要求應給予禮貌的拒絕。A 項表示“禁止”；C項表示“不必要”；而D項表示“不會”，均不符合題意。
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: 該題考查了could和be able to的區別，二者都可表示過去時間的能力，但如果表示過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
情態動詞shall在試題中表示征詢對方意見或請求指示。答句暗示 “沒有必要了”
二、考查情態動詞的推測用法
1、對現在或客觀事實的推測
對現在或客觀事實的肯定推測一般用must, will, should, may(might)或could，語氣依次遞減。
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廣東)
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的語氣最強，譯為“肯定”、“準是”、“想必是”；should的語氣次之，譯為“很可能”、“應該”，指按常理推測；may(might)譯為“也許、大概”，表示把握不大的推斷，后面常跟but。could的語氣最弱，譯為“也許”、“可能”。
對現在或客觀事實的否定推測，常用cannot譯為“根本不可能”, 表示推測的語氣非常肯定, may not, might not或could not譯為“可能不”、“也許不”，表示推測的語氣不很肯定。
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全國卷)
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 表示推測時常用于疑問和否定句中，用于肯定句中常常表示理論上的可能性，用來表達事件或情況可能發生(但并不牽涉到是否真的會發生)。
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全國Ⅰ)
A. can’t B. must not C. won’t D. may not
分析：might just as well表示“最好……，還是……為好”，說明喜歡的可能性不大；根據后面的I’m sure可以看出，推測的語氣較強，故用can’t譯為“根本不可能”、“想必不會”，表示驚異、懷疑的感情色彩，常用于否定語氣較強時。
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：對現在或客觀事實的疑問推測, 用can或could，表驚異、懷疑、困惑、失望等感情色彩。
2、對過去事實的推測
對過去已發生事情的肯定推測常用 “must/may/might +have + v-ed”，“must+ have + v-ed”意思為“想必、準是、一定是”，語氣非常肯定且只能用于肯定句，“may/might+ have + v-ed” 意思為“也許、或許”表示推測的把握不大，只能用于肯定或否定句，不用于疑問句；對過去的否定推測常用 “can/could /may’might not + have + v-ed”，意思為“不可能/可能沒做過……”；對過去的疑問推測只用“can/could + have + v-ed”，意思為“可能已經……”。
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：根據后一分句判斷，前一分句應該表示肯定完成了工作，故用“must+ have + v-ed”意思為“想必、準是、一定是”，語氣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表推測時，不用于否定，needn’t have +v-ed 表示“本來沒有必要”，shouldn’t have +v-ed表示“本來不應該”，“can/could not + have + v-ed” 意思為“不可能做過……”。
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 在此表示現在對過去所發生事情的否定推測，“現在不可能走遠了”
3、對將來事實的推測
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：由題意和下句中的 “I’m not sure” 可知這段對話中存在一種可能性推測，might可以用來表示一種比較委婉的可能性判斷
三、考查情態動詞的虛擬用法
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：對過去發生事情的責備或遺憾應用“情態動詞+have + v-ed”，據此可排除答案A、C，而would +have +v-ed常用于過去的虛擬語氣中，should(ought to)+ have + v-ed”表示本應該做某事而實際上沒有做，含有責備的含義。其否定式表示某種行為不該發生但卻發生了。故答案為D。
例30：He paid for a seat, when he ______ have entered free. (2005山東)
A. could B. would C. must D. need
分析：“could+ have + v-ed” 用于肯定句中，表示本來能夠做成某事的但結果沒能做成，含有遺憾的意味，側重有能力而沒能做成。“他花錢買了座位，而他本來可以免費進去的”。
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表示責備，后一分句則表明有被魚吃掉的可能性。“might+ have + v-ed” 用于肯定句中，表示本來有可能做成某事但結果沒能做成，含有遺憾的意味，側重有某種可能性但沒能做成。
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” 表示本來不必做某事而實際上做了某事，無批評責備之意，只是惋惜而已。如You needn’t have watered the flowers, for it is going to rain.你本不需要澆花的，因為天就要下雨了。