情态动词有can could, may might, must, have to, shall should, will would, dare dared, need needed, ought to等。 情态动词无人称和数的变化;不能单独使用,必须与其后的动词原形构成谓语。
一、 can, could
1 表示请求和允许。
-----Can I go now?
----- Yes, you can. / No, you can’t.
此时可与may互换。在疑问句中还可用could,might代替,不是过去式,只是语气更委婉,不能用于肯定句和答语中。
---- Could I come to see you tomorrow?
---- Yes, you can.No, I’m afraid not.
2 表示能力体力、知识、技能。
Can you lift this heavy box?体力
Mary can speak three languages.知识
Can you skate?技能
此时可用be able to代替。Can只有一般现在时和一般过去式;而be able to则有更多的时态。
I’ll not be able to come this afternoon.
当表示“经过努力才得以做成功某事”时应用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 表示客观可能性客观原因形成的能力。
They’ve changed the timetable, so we can go by bus instead.
This hall can hold 500 people at least.
4 表示推测惊讶、怀疑、不相信的态度,用于疑问句、否定句和感叹句中。
Can this be true?
This can’t be done by him.
How can this be true?
二、 may, might
1 表示请求和允许。might比 may语气更委婉,而不是过去式。否定回答时可用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...?征徇对方许可时比较正式和客气,而用Can I...?在口语中更常见。
2用于祈使句,表示祝愿。
May you succeed!
3 表示推测、可能性不用于疑问句。
might不是过去式,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 表示必须、必要。
You must come in time.
在回答引出的问句时,如果是否定的,不能用mustn’t禁止,不准,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是说话人的主观看法, 而have to则强调客观需要。Must只有一般现在时, have to 有更多的时态形式。
1. he play isn’t interesting, I really must go now.
2. I had to work when I was your age.
3 表示推测、可能性只用于肯定的陈述句
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作情态动词用时, 常用于疑问句、否定句和条件从句中, 过去式形式为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 作情态动词用时, 常用于疑问句、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作实义动词用时, 有人称、时态和数的变化。在肯定句中,dare后面常接带to的不定式。在疑问句和否定句中,dare后面可接带to或不带to的不定式。而need后面只能接带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.