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。
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。
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.