情態動詞的用法
情態動詞有can (could), may (might), must, have to, shall (should, will (would), dare (dared), need (needed), ought to等。 情態動詞無人稱和數的變化;不能單獨使用，必須與其後的動詞原形構成謂語。
一、 can, could
1) 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，不能用於肯定句和答語中。
---- Could I come to see you tomorrow?
---- Yes, you can. ( No, I’m afraid not. )
2) 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只有一般現在時和一般過去式;而be able to則有更多的時態。
I’ll not be able to come this afternoon.
當表示“經過努力才得以做成功某事”時應用be able to，不能用Can。如：
He was able to go to the party yesterday evening in spite of the heavy rain.
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、不相信的態度)，用於疑問句、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...?徵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)用於祈使句，表示祝願。
May you succeed!
3) 表示推測、可能性(不用於疑問句)。
might不是過去式，它所表示的可能性比may小。
may /might be very busy now.
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則強調客觀需要。Must只有一般現在時， have to 有更多的時態形式。
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(只用於肯定的陳述句)
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作情態動詞用時, 常用於疑問句、否定句和條件從句中, 過去式形式爲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 作情態動詞用時, 常用於疑問句、否定句。在肯定句中一般用must, have to, ought to, should代替。
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、時態和數的變化。在肯定句中，dare後面常接帶to的不定式。在疑問句和否定句中，dare後面可接帶to或不帶to的不定式。而need後面只能接帶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 用於第一人稱，徵求對方的意見。
What shall we do this evening?
2) shall 用於第二、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、願望和決心。
1. I will never do that again.
2. They asked him if he would go abroad.
3) would表示過去反覆發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“應該”，ought to表示義務或責任，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)。
must be home by now. (斷定他已到家)
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)
八、 情態動詞+不定式完成式(have done)
1) can / could + have done在肯定句中表示“本來可以做而實際上能做某事”，是虛擬語氣;在疑問句或否定句中表示對過去行爲的懷疑或不肯定, 表示推測。
1. You could have done better, but you didn’t try your best. (虛擬語氣)
2. He can’t have been to that town.(推測)
3. Can he have got the book?(推測)
2) may / might +不定式完成式(have done)
表示對過去行爲的推測。不能用於疑問句中，沒有虛擬語氣的用法。Might所表示的可能性比may小。
1. He may not have finished the work .
2. If we had taken the other road, we might have arrived earlier.
3)must +不定式完成式(have done)
用於肯定句中，表示對過去行爲的推測。意爲“一定、想必”。其疑問、否定形式用can，can’t代替。參看1) can / could + have done表示推測。
1. You must have seen the film Titanic.
2. He must have been to Shanghai.
4)should +不定式完成式(have done)
用於肯定句中，表示對過去行爲的推測。
He should have finished the work by now。
表示“本應該做而實際上沒有做某事”，其否定式表示某種行爲本不該發生卻發生了。可以與ought to +不定式完成式(have done)互換。
1. You ought to / should have helped him. (but you didn’t.)
2. She shouldn’t have taken away my measuring tape, for I wanted to use it.
5) needn’t +不定式完成式(have done)
表示“本來不必做而實際上做了某事”。
You needn’t have watered the flowers, for it is going to rain.
6) will +不定式完成式(have done)
主要用於第二、三人稱，表示對已完成的動作或事態的推測。
He will have arrived by now.
