情態動詞顧名思義就是表示情感和態度的詞，常見的有：can (could); may (might); must; have to; shall (should); will (would); need; dare (dared); ought to; 情態動詞沒有人稱和數的變化，不能單獨使用，它與其后的動詞原形合成謂語結構。它們的用法如下：
一. can和could 的用法
1. 表示能力或客觀的可能性，還可以表示請求和允許。如：
Can you finish this work tonight? Man cannot live without air.
Can I go now? Yes, you can.
注意：
1) could表示語氣較委婉的請求，主要用語疑問句，(用于此意義時不用在肯定句)答語用can。
Could I come to see you tomorrow?
Yes, you can. (不能用could). (否定句常用No, Im afraid not.)
2) can表示能力時，還可以用 be able to 代替，例如：
Ill be able to come this afternoon.
但當我們要表示某件事已成時，須用 was (were) able to不能用could，例如：
He was able to go to the party yesterday evening and he enjoyed himself very much.
2. 表示驚異、懷疑、不相信的態度。(主要用在否定句、疑問句和感嘆句中)
Can this be true? How can you say like that? This cannot be done by him.
3. can (could) + have + done 的疑問或否定形式表示對過去發生的行為懷疑或不肯定。
He cannot have been to that town. Can he have got the book?
二. may 和might 的用法
1. 表示許可，表示請求、允許時，might 比may 的語氣更加委婉一些，否定回答時可用cant 或mustnt, 表示不可以、禁止、阻止之意。 例如：
Might I use your pen? No, you mustnt.
May I take this book out? Yes, you can ( No, you cant/ mustnt.)
用May I 征詢對方許可在文體上較正式，在口氣上比較客氣，在日常口語中，用Can I 征詢對方意見在現代口語中更為常見。
2. 用于表示祝愿的句子中。例如：
May you success! May you be happy! May you have a good journey!
3. 表示推測、可能(疑問句中不用于此意)
He may be very busy now. (此句中用might 語氣較弱)
4. May (Might) + have + done 表示對過去發生的行為的推測。例如：
He may not have finished the work. (此句中用might 語氣較更弱)
三. must 和 have to 的用法
1. 表示必須、必要。
You must come on time.
當must 引出的問句時， 若是否定的回答，不能用mustnt，而用neednt或dont have to.
Must we hand in our exercise our books today?
Yes, you must. (No, neednt/you dont have to.)
2. must be + 表語的結構表示推測，它的否定或疑問形式用can 代替must。如：
He must be our new teacher.
Can he be our new teacher? He cant be our new teacher.
※ 附加疑問句要和謂語動詞實際時態一致。如：
He must be our new teacher, isnt he?
3. must + have +done 的結構常用在肯定句中，表示對過去或完成的情況推測，它的否定或疑問形式也用can代替must.
He must have finished his work.
※ 附加疑問句也要和謂語動詞實際時態一致。如：
He must have finished his work already, hasnt he?
He must have finished his work at four yesterday, didnt he?
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有下列幾點不同：
1) must表示的是說話人的主觀看法，而have to則往往強調客觀需要，例如：
The play is not interesting. I really must go now.
I had to work when I was your age.
2) 兩者的否定意義大不相同。例如：
You mustnt go. (你不可以去) You dont have to go. (你不必去)
3) 詢問對方的意見時應用must。
Must I hand in the exercise book before eight?
4) must 表示偏偏
Must it rain today? (今天怎么偏偏下雨呢?)
四. dare 和need的用法
1. need表示需要，必須，作情態動詞時，僅用于疑問句和否定句，在肯定句中一般用must, have to, ought to,或should代替。例如：
You neednt come so early.
Need I clean all the room right now? Yes, you must. ( No, you neednt.)
注意：neednt + have +done表示本來不必要做的事而實際上做了，例如：
You neednt have waited for me. 你本來沒有必要等我的。
2. dare作情態動詞時，主要用于疑問句，否定句和條件句中，一般不用于肯定句，例如：
How dare you take my book without permission?
He dare not speak English before such a crowd, dare he?
3. need和dare作情態動詞時，過去時形式與現在形式相同(美國英語中可用dared)。
He was so angry that no one dared say a word.
They said that he neednt stay there any longer.
4. need和dare常用實義動詞，有時態、人稱和數的變化。不同的是dare作實義動詞時，在肯定句中通常接帶to的不定式，在疑問句和否定句中，dare之后的不定式可帶to或不帶to。例如：
He needs to finish the work before nightfall.
I dare to swim across this river.
He does not dare (to) answer the teachers question.
Dont you dare (to) touch it?
I wonder he dared say that.
5. I dare say是習慣用語，常可寫為I daresay，用法有以下幾種：
1) 敢于說，通常接that或what引導的名詞性從句。
I dare say that he has stolen Toms football. I dare say what I think.
2) 我想;恐怕;大概;也許是，通常作插入語用，使句子委婉。
I dare say there is something wrong with the machines.
Youre tired, I dare say.
You, I dare say, think otherwise.
3) I dare say往往也作為反語使用。
Oh, you mean to defeat our football team? I dare say you will. (噢!你們想贏我們的足球隊?也許可以吧。)
五. shall和should的用法。
1. shall用于第一人稱疑問句。表示征求對方意見。
What shall we do this evening?
Shall I open the window?
2. shall 用語第三人稱疑問句中，表示說話人征求對方的意見或向對方請示。
When shall he be able to leave the hospital?
3. shall用于第二、三人稱的陳述句，表示說話人給對方的'命令、警告、允諾或威脅。
You shall fail if you dont work harder.
He shall have the book when I finish reading.
He shall be punished.
4. should表示勸告、建議、命令，其同義詞是ought to，在疑問句中，通常用should代替ought to.
You should go to class right away.
Should I open the window?
5. should + have +done應該做的事實上沒做;shouldnt have done不該做的事卻已經做了，常含有責備的意思。
You should have started earlier this morning.
You shouldnt have treated her like that.
六. will和would的用法。
1. 用于疑問句表示請求、建議等，would比will的語氣更加委婉。
Will you please give me a hand?
Would you mind my smoking here?
2. 表示意志、愿望和決心。
I will never do that again.
They asked us if we would do that again.
3. 用will be和will have done的結構表示推測，主要用于二、三人稱，前者表示對目前的情況的推測，后者表示對已經完成的動作或事態的推測。
This will be the book you want.
He will have arrived by now.
4. would可表示過去反復發生的動作或某種傾向，表示過去習慣時比used to正式，同時它沒有現在已無此習慣的含義。
I would visit him during my vacation.
The patient would not recover.
5. 表示料想或猜想。
He would be your new teacher.
He would not come any more.
七. ought to的用法。
1. ought to表示應該時意義與should基本一致，但更側重于一種義務或責任。
You ought to take care of him.
Parents ought to bring up their children.
2. 表示推測，注意與must表示推測時的區別。
He must be home by now.
He ought to be home by now.
This is where she must be.
This is where she ought to be.
3. ought to have done的用法與should have done的用法相同
注意：ought to在美國英語中，用于否定句和疑問句時，to可以省略。如：
Ought you smoke so much?
You oughtnt smoke so much.
八. used to; had better; would rather的用法。
1. used to表示過去習慣動作或狀態，現在已經不存在，在間接引語中，其形式可不變。如：He told us he used to play football when he was young.
1) 在疑問句、否定句或否定疑問句中，可有兩種形式。
疑問句：Did you used to/Used you to go to the same school as your brother?
否定句：I usednt to/didnt use to go there. (usednt 也可為usent, 發音不變)。
否定句疑問句：Usednt you to/Didnt you used to be interested in the play?
2) 在附加疑問句和簡略答語中，也可有兩種形式。
She used to be very fat, didnt/usednt she?
Did you used to/Used you to play chess? Yes, I did/used to.
2. had better意為最好，后接不帶to不定式。
We had better go now.
Hadnt we better stop now?
I think Id better be going.
You had better have done that.
3. would rather意為寧愿，表示選擇，后接不帶to的不定式。
Id rather not say anything.
Would you rather work on a farm?
Wouldnt you rather stay here? No, I would not. Id rather go there.
由于would rather表選擇，故后面可接than.
I would rather watch TV than go to see than go to see the film.
I would rather throw it away than you should get it.(若than后面接從句常用should do)
Id rather you didnt talk about this to anyone.
※ would rather 還可寫為would sooner/would as soon.
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