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1、情态动词有 can （could）, may （might）, must, have to, shall （should, will （would）,dare （dared）, n eed （n eeded）, 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, could1）表示能力（体力、知识、技能）。Can you lift this heavy box?（体力）Mary can speak three Ian guages.（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
2、式；而 be able to 则有更 多的时态。I ll not be able to come this after noon.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 eve ning in spite of the heavy rain.2）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。Coul
3、d I come to see you tomorrow?Yes, you can. （ No, I m afraid not.）3）表示客观可能性（客观原因形成的能力）。They ve cha nged the timetable, so we can go by bus in stead.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, might1）
4、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cant或must nt，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
5、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 to1）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neednt, 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
6、人的主观看法，而have to则强调客观需要。Must只有一般现在时，haveto有更多的时态形式。1. he play isn t interesting, I really must go now.2. I had to work whe n 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 wait ing for you now.四、dare, n eed1）dare作情态动词用时，常用于疑
7、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 fin
8、ish 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 n eeds to finish his homework today.五、shall, should1）shall用于第一人称，征求对方的意见。
9、What shall we do this eve ning?2）shall 用于第二、三人称，表示说话人给对方的命令、警告、允诺或威胁。1. You shall fail if you don t work hard.（ 警告）2. He shall have the book whe n I fin ish it.（允诺）3. He shall be puni shed.（威胁）六、will, would1）表示请求、建议等，would更委婉。Will / Would you pass me the ball, please?2）表示意志、愿望和决心。1. I will n ever do
10、 that aga in.2. They asked him if he would go abroad.3）would表示过去反复发生的动作或某种倾向。would表示过去习惯时比 used to正式,且没有“现已无此习惯”的含义。1. During the vacation, he would visit me every other day.2. The wound would not heal.4）表示估计和猜想。It would be about ten o clock when she left home.七、should, ought to1）should, ought to表示“
11、应该”，ought to 表示义务或责任，比 should语气重。1. I should help her because she is in trouble.2. You ought to take care of the baby.2）表示劝告、建议和命令。 should, ought to可通用，但在疑问句中常用should 。1. You should / ought to go to class right away.2. Should I open the win dow?3）表示推测should , ought to （ 客观推测）,must（ 主观推测）。1. He must
12、be home by now.（ 断定他已到家）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）八、情态动词+不定式完成式（have done）1）can / could + have done在肯定句中表示“本来可以做而实际上能做某事，是虚拟语气；在疑问句或否定句中表示对过去行为的怀疑或不肯定，表示推测。1. You could have done better, but you did
13、n t try your best.（虚拟语气）2. He can t have been to that town.（推测）3. Can he have got the book?（推测）2）may / might + 不定式完成式（ have done）表示对过去行为的推测。不能用于疑问句中，没有虚拟语气的用法。Might所表示的可能性比may小。1. He may not have fini shed the work .2. If we had taken the other road, we might have arrived earlier.3) must +不定式完成式(hav
14、e done)用于肯定句中，表示对过去行为的推测。意为“一定、想必”。其疑问、否定形式用can， can t 代替。参看 1) can / could + have done表示推测。1. You must have see n the film Tita nic.2. He must have bee n to Shan ghai.4) should + 不定式完成式(have done)用于肯定句中，表示对过去行为的推测。He should have fini shed the work by now。表示“本应该做而实际上没有做某事”，其否定式表示某种行为本不该发生却发生了。可以与oug
15、ht to + 不定式完成式(have done )互换。1. You ought to / should have helped him. (but you didn t.)2. She should n t have take n away my measuri ng tape, for I wan ted to use it.5) needn t + 不定式完成式( have done)表示“本来不必做而实际上做了某事”。You needn t have watered the flowers, for it is going to rain.6) will + 不定式完成式(have done)主要用于第二、三人称，表示对已完成的动作或事态的推测。He will have arrived by now.
