3)表“允许”。可表示委婉客气的提出问题或陈述看法

Could I use your bike?

Yes, you can.

他会记得那时吗?

I’m afraid I couldn’t give you an answer today.

恐怕我今天不能回答你。 

The teacher said you could go to the store for sweets.

老师说你可以去商店买糖。

3)Could/can+have done 结构表示对过去发生的事情的“怀疑”或“不肯定”。 could 加完成式还用于肯定句时一般表过去可能完成而却未完成的动作。

Can they have won the basketball match？

他们赢了那场篮球赛吗？

What you referred to just now can have made her very sad.

你刚刚所谈到的可能令他很伤心。

You could have completed the task a little earlier.

你本来能早点完成任务的。（但事实上并没有提前完成任务）

I could have passed my examination easily but I made too many stupid mistakes.

我本可以轻易通过考试，但我犯了太多不该犯的错误。

如表具体做某一件事的能力时，则须用 be able to .

He was able to translate the article without a dictionary. 

他可以不用词典翻译那篇文章。

Can表示一贯的能力 ，be able to表示客观能力和通过努力可以达到的能力

I can’t swim. But I am sure I will be able to swim through more practicing.

The fire spread through the hotel, but everyone was able to get out

When the boat sank he was able to swim to the bank

3.may 的用法 

1)表示请求、可以、允许。

You may drive the tractor. 你可以开那台拖拉机。

2)当回答由may 引起的问题时，否定答语要用must not,表示“不许可”、“不应该”、“不行”。

May I come in? 

Yes, you may.

No,you can’t 

No, you may not .

No ,you mustn’t

No ,you’d better not.

3) may /might 推测性用法 可能

He may be right.

He may not come today  (可能不)

He may /might come tomorrow.

, 注意: 1只用于肯定和否定句中,不用于疑问句中。

2 might 比may可能性更小

He might get a job.

He may get a job.

3 may no 可能不  can not不可能

He may not come

 He can’t come

3)表建议（可和as well 连用）

You may（might）as well stay where you are.

你还是原地待着好。（may as well 有“还是……的好”的含义）