英语基础语法：情态动词

情态动词主要说明说话者对某一行为的态度和观点，包括“揣测、判断、需要、能力、可能、许可和各种意愿”等。

can, could表示“能力”；can, could, may, might表示“许可”；must, have to, should, ought to表示“义务、责任”；shall, will, would表示“意愿、征求意见”；情态动词表示推测。

情态动词的两个特征：词义不完整，不能单独做谓语；没有人称和数的变化（have to例外）。

学习情态动词的难点：每个情态动词都有多个意义，要根据句意和语境来判断；有些特例和规定要着重理解。具体要求：①词义辨析；②情景中的运用；③情态动词+have+过去分词；④情态动词习惯用法；⑤反意疑问句。

一、can, could

1. 表示能力

—Can you drive? —Yes, I can.

What he said could not be heard by the students.

2. 表示（征求）许可或客观条件允许

Nobody can smoke here.

—Could I have a look at another pair? —Of course you can./No, I’m afraid you can’t.

3. 表示可能性、后接“be (get, seem, become)＋形容词”表示“有时候会”。

Anyone can make mistakes.

London can be very cold.

4. 表示说话人的推测、怀疑、惊奇、或不肯定，多用于否定句、疑问句和感叹句。could可以用于肯定句。

The news can not be true.

The essay can’t be finished in such a short time.

Can this be true?

How can/could you be so careless!

The injured could be in hospital now.

—Or they could be in one of those empty cottages near the pine trees…

—They can’t be there. People would have looked there first, I’m sure.

5. 如果对过去情况推测判断的句子是疑问句和否定句，用can／could和can’t／couldn’t．(may/ might have done也用于对已经发生过的情况的肯定推测判断，语气稍弱，意思是“可能，或许…”,一般不用于疑问句)。例如：

He can’t have gone abroad; I saw him an hour ago. 他不可能去国外，我一个小时前还见到他呢。

—Can she have passed the exam? — No, she can’t have. She didn’t work hard.

他通过考试了吗？不，他不可能通过考试。他学习不努力。

—They might have fallen behind and taken the wrong way.

—Maybe. Or they could have gone exploring on their own and lost their way back to the group.

二、may, might

1. 表示询问许可或允许

—May/Might I use your computer? —Of course you can/Sorry, I’m afraid you can’t.

2. 表示猜测，“可能、或许”。might可以表示过去、现在、将来的可能性（可能性较小）。may不能用于疑问句表可能性。

She may not call you back.

We may move abroad to live next year.

3. may/ might have done用于对已经发生过的情况的肯定推测判断，语气比较弱，意思是“可能，或许…”。例如：

She didn’t get there in time. She may have missed the bus.

He didn’t come to school today. He might have been ill.　

They may have been separated from the other students and…

4. might have done还可表示对过去可能发生而实际上没有发生的事情。

You drove too fast. You might have killed yourself.

5. may well +动词原形“（完全）能,很可能”。 may as well/might as well+动词原形:“最好…,倒不如…”=had better。

She has changed so much that you may well not recognize her.

You may as well do it in another way.

但I might as well be dead, for all you know. (即使我或许会死去，你也并不在意。本句中for all sb.knows/cares主要用于口语，意思是“（即使...）某人并不知道/在意”。口语就是这样，习以为常就好了。)

三、must与have to

1. must表示“必须”，强调说话人的看法，可表示过去、写作或将来，have to强调客观需要，有不同的时态、语态。must表示现在、将来的“义务”； 表示过去的义务用have to

2. mustn’t表示“禁止”，needn’t、don’t need to、 don’t have to表示“没必要”。回答must引起的问句时，否定回答用don’t need to、 don’t have to．

3. must可表示“偏偏”之意，指令人不悦。

The car must break down when I was about to start.

John, look at the time. Must you play the piano at such a late hour?

4. must还可表示必然性。

If you don’t hurry, you must miss the first bus.

5. 表示推测，“肯定是、准是”。“must+动词原形”表示对现在的推测；must be doing sth.表示对现在和将来正在的推测; must have done表示对过去的推测。

四、need和dare

      这两词既可做实义动词用，又可做情态动词用。作为情态动词，两者都用于疑问句，否定句和条件句，很少用于肯定的陈述。need 作实义动词时后面的不定式必须带to，而dare 作实义动词用时, 后面的to 时常可以被省略。

①need用作实义动词和其他普通的实意动词一样有时态、人称和数的变化，其否定式、疑问式要用do. need（需要, 要求）　need + n. / to do sth.

We need help him. (×)

②情态动词：need,只用原形need后加do，否定形式为need not。

——Need you go yet? ——Yes, I must. / No, I needn't.

He needs not leave at once. (×)

He needed not leave at once. (×)

——Need/Must I take the bus? ——Yes, you must.

③need用作实意动词的被动含义：need, want, require, worth（形容词）后面接doing 可表示被动：

sth. needs doing= sth.needs to be done.

④There is no need (for sb.)to do sth. 与It is no need doing sth.

⑤needn’t have done sth.与didn’t need to do sth.

They needn’t have pushed him so hard. (本来没必要但实际上逼迫他过分了)

They didn’t need to push him so hard. （实际上也没有逼他过分）

五、will和would

1. will表示主观意志、意愿，用于各人称。

If you’ll tell me about what you have in mind, I will help you.

2. 表示经常性、习惯性、倾向性，“经常、总是”。would用于表示过去的习惯。

Oil will float on water.

The door won’t open.

3. 表示命令或允诺

You will get necessary help.

4. 表示推测、估计，“可能、大概”，指现在或将来。可能性不如must大。would指现在或过去。

That will be my son knocking at the door. He said he would come here at this time.

That would be the pen you had been looking for.

5. 表示功能

The hall will seat over 300 people.

6. Will you…?　Would you like…? 表示肯定含义的请求、劝说时一般用some。

例如：Would you like some cake?

7. 否定结构中用will，一般不用would, won't you是一种委婉语气。

Won't you sit down?

would用于第二人称表示请求或征求意见，比will更客气。

六、shall, should

1. shall用于第一、第三人称疑问句，征求对方意见

2. shall用于第二、第三人称陈述句，表示命令、警告、威胁、允诺等。

He shall end up in prison if he steals like this.

3. 用于所有人称，表示规定、法令、预言等，意思为“必须”。

4. should（ought to）用于表示义务、责任、劝告、建议、命令等，“应该”

5. should have done sth.“本来应该…” shouldn’t have done sth.“本来不应该…”有时有责备的意思。

6. should表示有一定根据的推测或可能性，“应该会…”

He should have got there by now.

7. should在疑问句表示惊讶、不应该等。

8. 用于I should say/advise/think…,等，表示谦虚、婉转。（虚拟语气另讲）

七、had better (not) do sth.

had better (not) do sth. 最好（不）做某事。

八、易错点

1. used to do sth.过去常常做某事

be used to do sth.被/可以用来做某物

be used to doing sth.习惯于做某事

2. can与be able to

均表示“能力”。 be able to有各种时态变化，后不能跟不定式的被动形式。was/were able to do sth.表示过去经过努力做成了某事，不能用could代替was/were able to. could have done表示对过去情况的把握不大的肯定判断或过去本来有可能做但没有做。

3. used to do sth.表示过去的经常发生的动作或状况，暗示与现在不同，可用于无人称句和存在句。would do sth.仅表示过去习惯性的动作，没有与现在对比的含义，不能用于无人称句和存在句。

We would have a walk along the river bank after supper.

There used to be a temple on top of the hill.

4. 情态动词+have+过去分词

It must have rained last night, for the yard is wet this morning.

He can’t have gone left for Australia. I met him in the theater several minutes ago.

They could have gone exploring on their own and lost their way back to the group.

He may/might not have settled the problem.

5. 特定语气、态度：

can: 在否定、疑问、感叹句中表示惊异、怀疑

may: 表示祝愿

should: 表示惊奇、赞叹、惋惜、不满等情绪

must: 表示偏要、硬要

shall: 用于第二、第三人称陈述句，表示命令、警告、威胁、允诺

九、惯用法

1. cannot…too/enough 无论怎么…也不过分

2. cannot but do sth. 不得不…，只好…

3. may well do（原形）与may as well do sth. 见前面“二5”.

4. would rather do sth.(than do sth) 宁愿...(也不愿...)

would like sth.想要某物

would like to do sth. 想要做某事

prfer to do sth 宁愿做某事

prfer to doing sth.(to doing sth.)/would do sth than do sth./prefer to do sth rather than do sth.宁愿...(也不愿...)

prfer sb. to do sth/would rather sb. did sth. 宁愿sb.做某事