1） 情態(tài)動詞不能表示正在發(fā)生或已經(jīng)發(fā)生的事情，只表示期待或估計某事的發(fā)生。
2） 情態(tài)動詞除ought和have外，后面只能接不帶to 的不定式。
3） 情態(tài)動詞沒有人稱，數(shù)的變化，即情態(tài)動詞第三人稱單數(shù)不加-s。
4） 情態(tài)動詞沒有非謂語形式，即沒有不定式，分詞，等形式。
情態(tài)動詞有can (could), may (might), must, have to, shall (should, will (would), dare (dared), need (needed), ought to等。情態(tài)動詞無人稱和數(shù)的變化；不能單獨使用，必須與其后的動詞原形構(gòu)成謂語
一、 can, could
1) 表示能力（體力、知識、技能）。
Can you lift this heavy box? （體力）
Mary can speak three languages.（知識）
Can you skate? （技能）
此時可用be able to代替。Can只有一般現(xiàn)在時和一般過去式；而be able to則有更多的時態(tài)。
I’ll not be able to come this afternoon.
當表示“經(jīng)過努力才得以做成功某事”時應(yīng)用be able to，不能用Can。如：
He was able to go to the party yesterday evening in spite of the heavy rain.
2) 表示請求和允許。
-----Can I go now?
----- Yes, you can. / No, you can’t.
此時可與may互換。在疑問句中還可用could,
might代替，不是過去式，只是語氣更委婉，不能用于肯定句和答語中。
---- Could I come to see you tomorrow?
---- Yes, you can. ( No, I’m afraid not. )
3) 表示客觀可能性（客觀原因形成的能力）。
They’ve changed the timetable, so we can go by bus instead.
This hall can hold 500 people at least.
4) 表示推測（驚訝、懷疑、不相信的態(tài)度），用于疑問句、否定句和感嘆句中。
Can this be true?
This can’t be done by him.
How can this be true?
二、 may, might
1) 表示請求和允許。might比 may語氣更委婉，而不是過去式。否定回答時可用can’t
或mustn’t，表示“不可以，禁止”。
----Might/ May I smoke in this room?
---- No, you mustn’t.
---- May/Might I take this book out of the room?
---- Yes, you can. (No, you can’t / mustn’t. )
用May I...？征徇對方許可時比較正式和客氣，而用Can I...？在口語中更常見。
2）用于祈使句，表示祝愿。
May you succeed!
3) 表示推測、可能性（不用于疑問句）。
might不是過去式，它所表示的可能性比may小。
1．He may /might be very busy now.
2．Your mother may /might not know the truth.
三、 must, have to
1) 表示必須、必要。
You must come in time.
在回答引出的問句時，如果是否定的，不能用mustn’t（禁止，不準），而用needn’t, don’t have to（不必）.
---- Must we hand in our exercise books today?
---- Yes, you must.
---- No, you don’t have to / you needn’t.
2) must是說話人的主觀看法， 而have to則強調(diào)客觀需要。Must只有一般現(xiàn)在時， have to 有更多的時態(tài)形式。
1． he play isn’t interesting, I really must go now.
2． I had to work when I was your age.
3) 表示推測、可能性（只用于肯定的陳述句）
1． You’re Tom’s good friend, so you must know what he likes best.
2． Your mother must be waiting for you now.
四、 dare, need
1) dare作情態(tài)動詞用時, 常用于疑問句、否定句和條件從句中, 過去式形式為dared。
1． How dare you say I’m unfair?
2． He daren’t speak English before such a crowd, dare he?
3． If we dared not go there that day， we couldn’t get the beautiful flowers.
2) need 作情態(tài)動詞用時, 常用于疑問句、否定句。在肯定句中一般用must, have to, ought to, should代替。
1．You needn’t come so early.
2. ---- Need I finish the work today?
---- Yes, you must. / No, you needn’t.
3) dare和 need作實義動詞用時， 有人稱、時態(tài)和數(shù)的變化。在肯定句中，dare后面常接帶to的不定式。在疑問句和否定句中，dare后面可接帶to或不帶to的不定式。而need后面只能接帶to的不定式。
1. I dare to swim across this river.
2. He doesn’t dare (to) answer.
3. He needs to finish his homework today.
五、 shall, should
1) shall 用于第一人稱，征求對方的意見。
What shall we do this evening?
2) shall 用于第二、三人稱，表示說話人給對方的命令、警告、允諾或威脅。
1. You shall fail if you don’t work hard.(警告)
2. He shall have the book when I finish it.(允諾)
3. He shall be punished.(威脅)
六、 will, would
1) 表示請求、建議等，would更委婉。
Will / Would you pass me the ball, please?
2) 表示意志、愿望和決心。
1. I will never do that again.
2. They asked him if he would go abroad.
3) would表示過去反復(fù)發(fā)生的動作或某種傾向。would表示過去習慣時比used to正式，且沒有“現(xiàn)已無此習慣”的含義。
1. During the vacation, he would visit me every other day.
2. The wound would not heal.
4) 表示估計和猜想。
It would be about ten o’clock when she left home.
七、 should, ought to
1) should, ought to表示“應(yīng)該”，ought to表示義務(wù)或責任，比should語氣重。
1. I should help her because she is in trouble.
2. You ought to take care of the baby.
2) 表示勸告、建議和命令。should, ought to可通用，但在疑問句中常用should。
1. You should / ought to go to class right away.
2. Should I open the window?
3) 表示推測
should , ought to (客觀推測), must(主觀推測)。
1. He must be home by now. (斷定他已到家)
2. He ought to/should be home by now.(不太肯定)
3. This is where the oil must be. (直爽)
4. This is where the oil ought to/should be. (含蓄)
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