点击查看：2013年博思考试辅导：情态动词总结（一）   

1、 比较can 和be able to

1）can　could 表示能力；可能 (过去时用could),

只用于现在式和过去式(could)。be able to可以用于各种时态。

They will be able to tell you the news soon. 他很快就能告诉你消息了。

2）只用be able to

a. 位于助动词后。

b. 情态动词后。

c. 表示过去某时刻动作时。

d. 用于句首表示条件。

e. 表示成功地做了某事时，只能用was/were able to， 不能用could。

He was able to flee Europe before the war broke out.

= He managed to flee Europe before the war broke out.

注意：could不表示时态

1）提出委婉的请求，（注意在回答中不可用could）。

—— Could I have the television on?

—— Yes, you can. / No, you can't.

2）在否定，疑问句中表示推测或怀疑。

He couldn't be a bad man.

他不大可能是坏人。

2、比较have to和must

1)　两词都是'必须'的意思，have to 表示客观的需要， must 表示说话人主观上的看法，既主观上的必要。

My brother was very ill, so I had to call the doctor in the middle of the night.　我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)

He said that they must work hard. 他说他们必须努力工作。（主观上要做这件事）

2)　have to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。

He had to look after his sister yesterday.

3） 在否定结构中： don't have to　表示"不必"

mustn't　表示"禁止"，

You don't have to tell him about it.　你不一定要把此事告诉他。

You mustn't tell him about it.　你一定不要把这件事告诉他。

3、表示推测的用法

can, could, may, might, must 皆可表示推测，其用法如下：

1）情态动词+动词原形。

表示对现在或将来的情况的推测，此时动词通常为系动词。

I don't know where she is, she may be in Wuhan.

2）情态动词+动词现在进行时。

表示对现在或将来正在进行的情况进行推测。

At this moment, our teacher must be correcting our exam papers.

这时，我们老师想必在批改试卷。

3）情态动词+动词完成时。

表示对过去情况的推测。

We would have finished this work by the end of next December.

明年十二月底前我们很可能已完成这项工作了。

The road is wet. It must have rained last night.

地是湿的，昨天晚上一定下雨了。

4）情态动词+动词的现在完成进行时。

表示对过去正在发生事情的推测。

Your mother must have been looking for you.

你妈妈一定一直在找你。

5）推测的否定形式，疑问形式用can't, couldn't表示。

Mike can't have found his car, for he came to work by bus this morning.

迈克一定还没有找回他的车，因为早上他是坐公共汽车来上班的。

注意：could, might表示推测时不表示时态，其推测的程度不如 can, may。

4、 had better表示"最好"

had better 相当于一个助动词，它只有一种形式，它后面要跟动词原形。

had better do sth

had better not do sth

It is pretty cold. You'd better put on my coat.

She'd better not play with the dog.

had better have done sth表示与事实相反的结果，意为"本来最好"。

You had better have come earlier.

5、 will和would

注意：

1）would like； Would like to do = want to 想要，为固定搭配。

Would you like to go with me?

2）Will you…?　Would you like…? 表示肯定含义的请求劝说时，疑问句中一般用some, 而不是any。

Would you like some cake?

3）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?

6、 带to 的情态动词

带to 的情态动词有四个：ought to, have to, used to, be to, 如加上have got to ,(=must), be able to，为六个。它们的疑问，否定形式应予以注意：

Do they have to pay their bill before the end of the month?

She didn't use to play tennis before she was fourteen.

You ought not to have told her all about it.

Ought he to see a heart specialist at once.?

ought to 本身作为情态动词使用。其他的词作为实义动词使用，变疑问，否定时，须有do 等助动词协助。 典型例题

Tom ought not to ___ me your secret, but he meant no harm.

A.have told　B.tell　C.be telling　D. having told

答案A。由于后句为过去时，告诉秘密的动作又发生在其前因，此地应用过去完成时，但它在情态动词 ought to 后，所以用 have。

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