概 說(shuō)
助動(dòng)詞(auxiliary)主要有兩類：基本助動(dòng)詞(primary auxiliary)和情態(tài)助動(dòng)詞(modal auxiliary)?；局鷦?dòng)詞有三個(gè)：do, have和be；情態(tài)助動(dòng)詞有十三個(gè)：may, might; can, could; will, would; shall, should; must, need, dare, used to, ought to. 上述兩類助動(dòng)詞的共同特征是，在協(xié)助主動(dòng)詞構(gòu)成限定動(dòng)詞詞組時(shí)，具有作用詞的功能：
1） 構(gòu)成否定式：
He didn't go and neither did she.
The meeting might not start until 5 o'clock.
2） 構(gòu)成疑問(wèn)式或附加疑問(wèn)式：
Must you leave right now?
You have been learning French for 5 years, haven't you?
3） 構(gòu)成修辭倒裝：
Nowhere can he obtain any information about his sister.
Hardly had he arrived when she started complaining.
4） 代替限定動(dòng)詞詞組：
A: Who can solve this crossword puzzle?
B: Tom can.
A: Shall I write to him?
B: Yes, do.
情態(tài)助動(dòng)詞的特征
基本助動(dòng)詞與情態(tài)助動(dòng)詞最主要的區(qū)別之一是，基本助動(dòng)詞本身沒(méi)有詞義，而情態(tài)助動(dòng)詞則有自己的詞義，能表示說(shuō)話人對(duì)有關(guān)動(dòng)作或狀態(tài)的看法，或表示主觀設(shè)想：
What have you been doing since? （構(gòu)成完成進(jìn)行體，本身無(wú)詞義）
I am afraid I must be going. （一定要）
You may have read some account of the matter. （或許已經(jīng)）
除此之外，情態(tài)助動(dòng)詞還有如下詞法和句法特征：
1） 除ought和used以外，其他情態(tài)動(dòng)詞后面只能接不帶to的不定式。如果我們把ought to和used to看做是固定詞組的話，那么，所有情態(tài)動(dòng)詞無(wú)一例外地只能接不帶to的不定式：
We used to grow beautiful roses.
I asked if he would come and repair my television set.
2） 情態(tài)助動(dòng)詞在限定動(dòng)詞詞組總總是位居第一：
They need not have been punished so severely.
3） 情態(tài)助動(dòng)詞用于第三人稱單數(shù)現(xiàn)在時(shí)的時(shí)候，沒(méi)有詞形變化，即其詞尾無(wú)-s形式：
She dare not say what she thinks.
4） 情態(tài)動(dòng)詞沒(méi)有非限定形式，即沒(méi)有不定式、-ing分詞和-ed分詞形式：
Still, she needn't have run away.
5） 情態(tài)助動(dòng)詞的“時(shí)”的形式并不是時(shí)間區(qū)別的主要標(biāo)志。在不少場(chǎng)合，情態(tài)助動(dòng)詞的現(xiàn)在時(shí)和過(guò)去時(shí)形式都可以表示現(xiàn)在、過(guò)去或?qū)?lái)時(shí)間：
Would you mind very much if I ask you to do something?
She told him he ought not to have done it.
6） 情態(tài)助動(dòng)詞之間是相互排斥的，即在一個(gè)限定動(dòng)詞詞組中只能出現(xiàn)一個(gè)情態(tài)助動(dòng)詞，但有時(shí)卻可以與have和be基本助動(dòng)詞連用：
You should have washed the wound.
Well, you shouldn't be reading a novel.
現(xiàn)將情態(tài)助動(dòng)詞的各種形式列表如下：
情態(tài)助動(dòng)詞的意義和用法
情態(tài)助動(dòng)詞從最普通的意義上來(lái)說(shuō)，是修飾分句意義的一種方式，它可以反映說(shuō)話者對(duì)其表述是否真實(shí)和可能的程度作出判斷。但不同的情態(tài)助動(dòng)詞本身所包含的意義和用法又不同，下面逐個(gè)分析。
1） can和could的用法
1. 表示能力或客觀可能性，還可以表示請(qǐng)求和允許。如：
Can you finish this work tonight?
Man cannot live without air.
— Can I go now? — Yes, you can.
注意：①could也可表示請(qǐng)求，預(yù)期委婉，主要用于疑問(wèn)句，不可用于肯定句，答語(yǔ)應(yīng)用can（即could不能用于現(xiàn)在時(shí)態(tài)的簡(jiǎn)略答語(yǔ)中）。如：
Could I come to see you tomorrow?
Yes, you can. （否定答語(yǔ)可用No, I'm afraid not.）
②can表示能力時(shí)，還可用be able to代替。如：
I'll not be able to come this afternoon.
2. 表示驚異、懷疑、不相信的態(tài)度。（主要用在否定句、疑問(wèn)句或驚嘆句中）
Can this be true?
How can you be so careless!
This cannot be done by him.
3. “can(could) + have + 過(guò)去分詞”的疑問(wèn)或否定形式表示對(duì)過(guò)去發(fā)生的行為懷疑或不肯定。如：
He cannot have been to that town.
Can he have got the book?
2） may和might的用法
1. 表示許可。
表示請(qǐng)求、允許時(shí)，might比many的語(yǔ)氣更委婉一些，否定回答時(shí)要用mustn't表示“不可以”、“禁止”、“阻止”之意。如：
You may drive the car.
— Might I use your pen? — No, you mustn't.
用May I ... 征詢對(duì)方許可在文體上比較正式，在口氣上比較客氣。在日?？谡Z(yǔ)中，用Can I ... 征詢對(duì)方意見(jiàn)在現(xiàn)代口語(yǔ)中更為常見(jiàn)。
2. 用于祈使句中表示祝愿。如：
May you succeed!
3. 表示推測(cè)、可能（疑問(wèn)句不能用于此意）。
He may be very busy now.
4. “may(might) + have + 過(guò)去分詞”表示對(duì)過(guò)去發(fā)生的行為的推測(cè)。如：
He may not have finished the work.
3） must和have to的用法
1. 表示必須、必要。如：
You must come in time.
回答must引出的問(wèn)句時(shí)，如果是否定的回答，不能用mustn't，而要用needn't或don't have to。
— Must we hand in our exercise books today?
— Yes, you must. (No, you don’t have to.)
2. “must be + 表語(yǔ)”的結(jié)構(gòu)表示推測(cè)，它的否定或疑問(wèn)式用can代替must。
This must be your pen.
3. “must + have + 過(guò)去分詞”的結(jié)構(gòu)常用在肯定句中，表示對(duì)過(guò)去發(fā)生的行為的推測(cè)。它的否定或疑問(wèn)式用can代替must。
He must have been to Shanghai.
4. have to的含義與must相似，兩者往往可以互換使用，但have to有各種形式，隨have的變化而定。must與have to有下列幾點(diǎn)不同：
① must表示的是說(shuō)話人的主觀看法，而have to則往往強(qiáng)調(diào)客觀需要。如：
The play is not interesting. I really must go now.
I had to work when I was your age.
② must一般只表現(xiàn)在，have則有更多的時(shí)態(tài)形式。
③ 二者的否定意義不大相同。如：
You mustn't go. 你可不要去。
You don't have to go. 你不必去。
④ 詢問(wèn)對(duì)方的意愿時(shí)應(yīng)用must。如：
Must I clean all the room?
4） dare和need的用法
1. need表示“需要”或“必須”，作情態(tài)動(dòng)詞時(shí)，僅用于否定句或疑問(wèn)句中，在肯定句中一般用must, have to, ought to,或should代替。如：
You needn’t come so early.
— Need I finish the work today?
— Yes, you must.
注意：needn't + 不定式的完成式“表示本來(lái)不必做某事而實(shí)際上做了某事”。如：
You needn't have waited for me.
2. Dare作情態(tài)動(dòng)詞時(shí)，主要用于疑問(wèn)句、否定句和條件從句中，一般不用在肯定句中。如：
How dare you say I'm unfair.
He daren't speak English before such a crowd, dare he?
3. Dare和need常用作實(shí)義動(dòng)詞，有時(shí)態(tài)、人稱和數(shù)的變化，所不同的是，作實(shí)義動(dòng)詞時(shí)，在肯定句中，dare后面通常接帶to的不定式，在否定和疑問(wèn)句中，dare后面可接帶to或不帶to的不定式。如：
I dare to swim across this river.
He does not dare (to) answer.
Don't you dare (to) touch it!
I wondered he dare (to) say that.
He needs to finish it this evening.
5） shall和should的用法
1. Shall用于第一人稱，表示征求對(duì)方的意愿。如：
What shall we do this evening?
2. Shall用于第一、第三人稱疑問(wèn)句中，表示說(shuō)話人征求對(duì)方的意見(jiàn)或向?qū)Ψ秸?qǐng)示。如：
Shall we begin our lesson?
When shall he be able to leave the hospital?
3. Shall用于第二、第三人稱，表示說(shuō)話人給對(duì)方命令、警告、允諾或威脅。如：
You shall fail if you don't work harder. （警告）
He shall have the book when I finish reading. （允諾）
He shall be punished. （威脅）
4. Should表示勸告、建議、命令，其同義詞是ought to；在疑問(wèn)句中，通常用should代替ought to。如：
You should go to class right away.
Should I open the window?
Should的含義較多，用法較活，現(xiàn)介紹三種其特殊用法。請(qǐng)看下面的句子：
① I should think it would be better to try it again. 我倒是認(rèn)為最好再試一試。
② You are mistaken, I should say. 依我看，你是搞錯(cuò)了。
③ I should advise you not to do that. 我倒是勸你別這樣做。
④ This is something I should have liked to ask you. 這是我本來(lái)想問(wèn)你的。
從以上例句可以看出：情態(tài)動(dòng)詞should用于第一人稱時(shí)可以表示說(shuō)話人的一種謙遜、客氣、委婉的語(yǔ)氣。
Should還可以用在if引導(dǎo)的條件從句，表示一件事聽起來(lái)可能性很小，但也不是完全沒(méi)有可能。相當(dāng)于“萬(wàn)一”的意思。從句謂語(yǔ)由should加動(dòng)詞原形構(gòu)成，主句謂語(yǔ)卻不一定用虛擬語(yǔ)氣。如：
⑤ Ask her to ring me up if you should see her. 你萬(wàn)一見(jiàn)到她，請(qǐng)讓她給我打個(gè)電話。
⑥ If you should change your mind, please let us know. 萬(wàn)一你改變主意，請(qǐng)通知我們。
⑦ Should I (If I should) be free tomorrow I will come. 萬(wàn)一我明天有時(shí)間，我就來(lái)。
此外，Why(or How) + should結(jié)構(gòu)表示說(shuō)話人對(duì)某事不能理解，感到意外、驚異等意思。意為“竟會(huì)”。如：
⑧ Why should you be so late today? 你幾天怎么來(lái)得這么晚？
⑨ — Where is Betty living? — 貝蒂住在哪里？
— How should I know? — 我怎么會(huì)知道呢？
⑩ I don't know why you should think that I did it. 我真不知道你憑什么認(rèn)為這件事是我干的。
5. “should + have + 過(guò)去分詞”結(jié)構(gòu)一般表示義務(wù)，表示應(yīng)該做到而實(shí)際上沒(méi)有做到，并包含一種埋怨、責(zé)備的口氣。如：
She should have finished it.
I should have helped her, but I never could.
You should have started earlier.
6） will和would的用法
1. 表示請(qǐng)求、建議等，would比will委婉客氣。如：
Would you pass me the book?
2. 表示意志、愿望和決心。如：
I will never do that again.
They asked if we would do that again.
3. 用“will be”和“will(would) + have + 過(guò)去分詞”的結(jié)構(gòu)表示推測(cè)，主要用于第二、三人稱。前者表示對(duì)目前情況的推測(cè)，后者表示對(duì)已經(jīng)完成的動(dòng)作或事態(tài)的推測(cè)。如：
This will be the book you want.
He will have arrived by now.
The guests would have arrived by that time.
I thought you would have finished this by now.
4. Would可表示過(guò)去反復(fù)發(fā)生的動(dòng)作或某中傾向。Would表過(guò)去習(xí)慣時(shí)比used to正式，并沒(méi)有“現(xiàn)已無(wú)此習(xí)慣”的含義。如：
The wound would not heal.
During the vacation he would visit me every week.
5. 表料想或猜想。如：
It would be about ten when he left home.
What would she be doing there?
I thought he would have told you all about it.
7） ought to的用法
1. Ought to表示應(yīng)該。如：
You ought to take care of him.
2. 表示推測(cè)。注意與must表示推測(cè)時(shí)的區(qū)別：
He must be at home by now. （斷定他已到家）
He ought to be home by now. （不十分肯定）
This is where the oil must be. （比較直率）
This is where the oil ought to be. （比較含蓄）
3. “ought to + have + 過(guò)去分詞”表示過(guò)去應(yīng)做某事而實(shí)際未做。如：
You ought to have asked him (but you didn't).
這時(shí)，ought to和should可以互相換用。
注意：在美國(guó)英語(yǔ)中，ought to用于否定句和疑問(wèn)句時(shí)，to可以省略。如：
Ought you smoke so much?
You oughtn't smoke so much.
8） used to，had better，would rather的用法
1. Used to表示過(guò)去的習(xí)慣動(dòng)作或狀態(tài)，現(xiàn)在已不存在，在間接引語(yǔ)中，其形式可不變。如：
He told us he used to play football when he was young.
在疑問(wèn)句、否定句、否定疑問(wèn)句或強(qiáng)調(diào)句中，可有兩種形式：
疑問(wèn)句
Did you use to go to the same school as your brother?
Used you to go to the same school as your brother?
否定句
I usedn't to go there.
I didn't use to go there.
Usedn't 亦可拼作usen't，但發(fā)音皆為[ju:znt]。
否定疑問(wèn)句
Usen't you to be interested in the theatre?
Didn't you use to be interested in the theatre?
強(qiáng)調(diào)句
I certainly used to smoke, but it was a long time ago.
I certainly did use to smoke, but it was a long time ago.
其反意疑問(wèn)句或簡(jiǎn)略回答中，也有兩種形式：
She used to be very fat, didn't she? （正式）/ use(d)n't she? （口語(yǔ)）
Did you use to play chess? Yes, I did.
Used you to get up early in the morning? Yes, I did. (Yes, I used to.)
2. Had better意為“最好”，后接不帶to的不定式。如：
— We had better go now.
— Yes, we had (we'd better / we had better).
Hadn't we better stop now? (Had we better not stop now?)
I think I'd better be going. （用于進(jìn)行時(shí)態(tài)，表“最好立即”）
You had better have done that. （用于完成時(shí)態(tài)，表未完成動(dòng)作）
注：①had best與had better同意，但較少用。②You had better … 用于同輩或小輩，對(duì)長(zhǎng)輩不可用。
3. Would rather意為“寧愿”，表選擇，后接不帶to的不定式。如：
I'd rather not say anything.
Would you rather work on a farm?
— Wouldn't you rather stay here?
— No, I would not. I'd rather go there.
由于would rather表選擇，因而后可接than。如：
I would rather work on a farm than in a factory.
I would rather watch TV than go to see the film.
I would rather lose a dozen cherry trees than that you should tell me one lie.
I'd rather you didn't talk about this to anyone. （句中的'd rather不是情態(tài)動(dòng)詞，would在此是表愿望的實(shí)義動(dòng)詞）
高中情態(tài)動(dòng)詞的重難點(diǎn)突破
一.概念:
情態(tài)動(dòng)詞是表示能力,義務(wù),必須,猜測(cè)等說(shuō)話人的語(yǔ)氣或情態(tài)的動(dòng)詞.
二.相關(guān)知識(shí)點(diǎn)精講:
1．can
1)表能力
can表能力時(shí)意味著憑體力或腦力或技術(shù)等可以無(wú)甚阻力地去做某事。
I can climb this pole. 我能爬這根桿子。
He is only four , but he can read. 他只有4歲，但已認(rèn)得字了。
Fire can’t destroy gold. 火燒不毀金子。
因?yàn)閏an不能和其他助動(dòng)詞連用，所以表示將來(lái)式時(shí)用will be able to
You will be able to skate after you have practiced it two or three times.
你練習(xí)兩三次后就會(huì)溜冰了。
2)表可能性
多用于否定與疑問(wèn)結(jié)構(gòu)中，但也可用在肯定句中。
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？
can 用在肯定句中表示理論上的可能性（一時(shí)的可能）。
A horse 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(huì)不通的。
may 在肯定句中表示現(xiàn)實(shí)的可能性。
The road may be blocked. 這條路可能不通了。
3)表示允許（和may意思相近）常見(jiàn)于口語(yǔ)。
Can (May) I come in ? 我能進(jìn)來(lái)嗎？
Can I smoke here ? 我可以在這里抽煙嗎？
2.could的用法
1)表過(guò)去的可能和許可，（多用于間接引語(yǔ)中）
At that time we thought the story could not be true.
那時(shí)我們認(rèn)為所說(shuō)的事不可能是真的。
Father said I could swim in the river.
爸爸說(shuō)我可以在河里游泳。
2)表過(guò)去的能力
I could swim when I was only six.
我剛六歲就能游泳。
Could在肯定句中表示過(guò)去的能力時(shí)，常表抽象的一般的能力。
He could be very naughty when he was a child.
他小時(shí)候會(huì)是很頑皮的。
3)表“允許”?？杀硎疚窨蜌獾奶岢鰡?wèn)題或陳述看法
Could I use your bike?
Yes, you can.
他會(huì)記得那時(shí)嗎?
I’m afraid I couldn’t give you an answer today.
恐怕我今天不能回答你。
The teacher said you could go to the store for sweets.
老師說(shuō)你可以去商店買糖。
4)Could/can+have done 結(jié)構(gòu)表示對(duì)過(guò)去發(fā)生的事情的“懷疑”或“不肯定”。 could 加完成式還用于肯定句時(shí)一般表過(guò)去可能完成而卻未完成的動(dòng)作。
Can they have won the basketball match？
他們贏了那場(chǎng)籃球賽嗎？
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。
You could have completed the task a little earlier.
你本來(lái)能早點(diǎn)完成任務(wù)的。（但事實(shí)上并沒(méi)有提前完成任務(wù)）
I could have passed my examination easily but I made too many stupid mistakes.
我本可以輕易通過(guò)考試，但我犯了太多不該犯的錯(cuò)誤。
如表具體做某一件事的能力時(shí)，則須用 be able to .
He was able to translate the article without a dictionary.
他可以不用詞典翻譯那篇文章。
Can表示一貫的能力 ，be able to表示客觀能力和通過(guò)努力可以達(dá)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.may 的用法
1)表示請(qǐng)求、可以、允許。
You may drive the tractor. 你可以開那臺(tái)拖拉機(jī)。
2)當(dāng)回答由may 引起的問(wèn)題時(shí)，否定答語(yǔ)要用must not,表示“不許可”、“不應(yīng)該”、“不行”。
May I come in?
Yes, you may.
No,you can’t
No, you may not .
No ,you mustn’t
No ,you’d better not.
3) may /might 推測(cè)性用法 可能
He may be right.
He may not come today (可能不)
He may /might come tomorrow.
注意 : (1)只用于肯定和否定句中,不用于疑問(wèn)句中。
(2) might 比may可能性更小
He might get a job.
He may get a job.
(3) may no 可能不 can not不可能
He may not come
He can’t come
3)表建議（可和as well 連用）
You may（might）as well stay where you are.
你還是原地待著好。（may as well 有“還是……的好”的含義）
4)表祝愿
May you be happy!
might
1)表過(guò)去的“可能”和“允許”多用于間接引語(yǔ)。
She said that he might take her dictionary.
她說(shuō)他可以拿她的詞典去用。
除在間接引語(yǔ)中外，might一般不表示過(guò)去的“可能”與“許可”。表過(guò)去的“可能”可用could，表過(guò)去的“許可”可用were (was) allowed to。
2)表現(xiàn)在的“可能”，其可能性要比may小。
Electric irons could be dangerous; they might give you a severe shock.
電熨斗會(huì)有危險(xiǎn)，它可能電著人。
3)may (might) + have +done 表示對(duì)過(guò)去發(fā)生行為的推測(cè)，含有“想必”、“也許是”的意思。
It may have been true. 這事也許是真的。
He might not have settled the question. 他可能尚未解決那個(gè)問(wèn)題。
4.must 的主要用法。
1)表示必須、必要
We must do everything step by step .我們必須按部就班地做一切事情。
Why must you always bother me? 為什么你偏要打擾我呢。
2)must be + 表語(yǔ)的結(jié)構(gòu)，通常表示猜測(cè)，含有“一定”之意。（只用在肯定句中）
He must be an honest boy. 他一定是個(gè)誠(chéng)實(shí)的男孩。
This must be your room. 這一定是你的房間。
3)must 的否定式有兩個(gè)：當(dāng)回答由must引起的問(wèn)題時(shí)，否定答復(fù)要用needn’t或don’t have to 表示“不必”、“無(wú)須”、“用不著”、“不一定”的意義。當(dāng)表示“不應(yīng)該”、“不許可”、“禁止”時(shí)，就用must not。
Must I go tomorrow?明天我必須去嗎？
Yes, please.是的，請(qǐng)吧！
No , you needn’t. 不，你不必去。
4)must +have +過(guò)去分詞的結(jié)構(gòu)，常用在肯定句中，表示對(duì)過(guò)去發(fā)生行為的推測(cè)，含有“一定”、“準(zhǔn)是”的意思。否定和疑問(wèn)句用can。
She must have studied English before.她以前一定學(xué)過(guò)英語(yǔ)。
5.have to的含義與must是很接近的,只是have to 比較強(qiáng)調(diào)客觀需要，must著重說(shuō)明主觀看法。
I must clean the room.（主觀想法）
I have to clean the room.（客觀需要）
另外，have to 能用于更多時(shí)態(tài)：
We had to be there at ten .我們得在十點(diǎn)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。
have to 的否定式：don’t have to do 表示“不必做……”之意。
6.ought to 的用法
Ought to 后接動(dòng)詞原形，表義務(wù)，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(yīng)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(yīng)該抽這么多煙。
也可以用于疑問(wèn)句，如：
Ought you to smoke so much?你應(yīng)該抽這樣多煙嗎？
Ought to 在間接引語(yǔ)中表過(guò)去時(shí)形式不變，如：
He said you ought to tell the police.
他說(shuō)你應(yīng)該去報(bào)告警察。
7.shall的用法
1)用于第一人稱征求對(duì)方的意見(jiàn)，如：
What shall I wear on the journey? 我路上穿什么好呢？
Shall we dance? 我們跳舞好嗎？
2)shall 用于第二、三人稱時(shí)表允諾，警告，命令，威脅（現(xiàn)已少見(jiàn)），如：
She shall get her share. 她可以得到她的一份。
You shall have it back tomorrow.你明天可以將它拿回。
情態(tài)動(dòng)詞should一般不應(yīng)被認(rèn)為是情態(tài)動(dòng)詞shall的過(guò)去式，主要用法有：
1)用于第一人稱疑問(wèn)句中詢問(wèn)對(duì)方的意愿，但語(yǔ)氣較委婉溫和，如：
What should we do now? 我們現(xiàn)在該怎么辦？
2)表示應(yīng)該、必須，常與must 換用。例如：
We should (must) master a foreign language at least.
我們應(yīng)當(dāng)至少掌握一門外語(yǔ)。
3)“should+be+表語(yǔ)”的結(jié)構(gòu)，表示推測(cè)或驚奇。例如：
They should be back by now. 他們現(xiàn)在應(yīng)該回來(lái)了吧。
I am sorry that she should be so careless. 我感到遺憾她竟會(huì)那樣粗心。
4)“should+have+過(guò)去分詞”的結(jié)構(gòu)，表示過(guò)去該做而實(shí)際上尚未做的動(dòng)作或行為；其否定則表示發(fā)生了不應(yīng)該發(fā)生的行為。其同義結(jié)構(gòu)“ought to have +過(guò)去分詞”,表示過(guò)去“早應(yīng)該”、“本當(dāng)”之意，語(yǔ)氣較強(qiáng)。例如：
I should have thought of that. 這一點(diǎn)我是應(yīng)當(dāng)想到的。（但沒(méi)想到）
They should not have left so soon.他們不應(yīng)當(dāng)走得這么早。（但已走了）
5) 在“It is natural (strange, natural, necessary, surprised, impossible, important ) that……”句型中，主語(yǔ)從句中的謂語(yǔ)動(dòng)詞要用should +動(dòng)詞原形”表示“理所當(dāng)然”、“奇怪”、“必要”、“驚異”等的意思。在lest（以免）、for fear (that) (以防)、in case（以備萬(wàn)一）等之后也要用should+動(dòng)詞原形;在advise, sugest, order, demand, request 等的從句中should+do”例如：
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(huì)說(shuō)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。
