2020英语专八语法复习：情态动词的用法

　　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.

　　五、 shall, should

　　1) shall 用于第一人称，征求对方的意见。

　　What shall we do this evening?

　　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 when I finish it.(允诺)

　　3. He shall be punished.(威胁)

　　六、 will, would

　　1) 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 never do that again.

　　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 to

　　1) should, ought to表示“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 window?

　　3) 表示推测

　　should , ought to (客观推测), must(主观推测)。

　　1.He must 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 didn'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 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 seen the film Titanic.

　　2. He must have been to Shanghai.

　　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 finished 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ought 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 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.

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