比较can 和be able to

　1）can/could 表示能力；可能, 只用于现在式和过去式（could）。be able to可以用于各种时态。例如：

　 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的情况：

　 a. 位于助动词 / 情态动词后。

　 b. 表示成功地做了某事时，用was/were able to，不能用could。例如：

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

他在战争爆发之前逃离欧洲。

注意：could有时不表示时态

　1) 提出委婉的请求，（注意在回答中不可用could）。例如：

　　 --- Could I have the television on? 我能看电视吗？

　　 --- Yes, you can. / No, you can't. 可以/不可以。

　2) 在否定句、疑问句中表示推测或怀疑。例如：

　　 He couldn't be a bad man. 他不大可能是坏人。　

2. may/ might: 1）表示允许或请求；2）表示可能或推测；3）may 放在句首，表示祝愿。

May I go home now? (permission)

If you lose too much blood, you may die. (possibility)

He might be at home.

May God bless you!

* May not "可能不" can not "不可能"

3. must / have to：1）必须 2）推测

　否定结构中：don't have to=need not 表示"不必"，mustn't表示"禁止"。例如：

　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.　　　　你不得把这件事告诉他。

　have to / have got to 的用法

must: 偏偏 Why must it rain on Sunday?

Listen, there must be some children in the room.

4. 表示推测的用法

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

1) 对现在情况的推测: 情态动词+动词原形，此时动词通常为系动词。

肯定：must/may/might + be sth. /do sth. /be doing sth.

否定：can/could not + be sth./do sth./ be doing sth

疑问：Can/Could + be sth./do sth./ be doing sth

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

　2）对过去情况的推测。

肯定：must/may/might +have been /done

否定：can/could not + have been /done

疑问：Can/Could + have been /done

Eg: The road is wet. It must have rained last night. 地是湿的，昨天晚上一定下雨了。

　3）对过去正在发生事情的推测: 情态动词+动词的现在完成进行时。

Eg: Your mother must have been looking for you. 你妈妈一定一直在找你。

　4) 注意其反意疑问句的形式：

He must be at home, isn't he?

They must be in the classroom, aren't they?

He may have watched TV yesterday, didn't he?

She must have finished her homework, hasn't she?

5. will /would: