can和could

1.表示能力，可译为“能，会”。

I can swim.我会游泳。

Emily can dance well and her mother could dance well when she was young.

艾米丽舞跳得很好，她妈妈年轻时舞跳得也很好。

The cinema can seat 1,000 people.

这电影院能容纳1，000人。

I could not read such an easy book when I was 7 years old.

我七岁时连如此简单的书我也看不懂。

2.表示允许、许可，常用在口语中。could比can语气上要客气。

--- Could I use your dictionary

我可以用一下你的字典吗?

---Yes, go ahead.

可以，用吧。(或Yes, you can.但不能说Yes, you could.)

Could Can you tell me how to get to the zoo

劳驾，你能告诉我怎么去动物园吗?

He asked me whether he could take the book out of the reading room.

他问我可不可以把书带出阅览室。

3.表示推测，可用于肯定句(can罕见)、否定句和疑问句，但更常见于疑问句或否定句。在此种用法中can和could没有时态的区别，只是表示可能性的大小，can表示推测的可能性比could大。

Can he be ill at home

他会是生病在家吗?

Can the story be true

这个故事会是真的吗?

He cannot be at home.

他不可能在家。

You mustn't smoke while you are walking around in the woods. You could start a fire.

在林子里走时一定不要吸烟，那样可能会引起火灾。

4.表示惊异、怀疑、不相信等态度，主要用于否定句和疑问句中。

How can you be so careless

你怎么这么粗心?

Where can could they have gone

他们会去了哪里?

He can'tcouldn't be over sixty.

他不可能超过60岁。

5.Could可以用在虚拟条件句中。

If I could fly, I should be very happy.

如果我能飞，我会很高兴。

If you had followed my advice, you could have finished it.

如果你当时听我的，你早就做完了。

If you had tried harder, you could have passed your exam.

如果你再努力些，你就能通过考试了。

6.can与be able to的区别

①can表示“能力”时，和be able to相当，许多场合都可以互相替换。但当叙述过去经过一番努力才能完成的事情或前面有特殊说明，表示你有能力时，只能用be able to。

【正】Can you speak any foreign languages

你会说外语吗?

【正】Are you able to speak any foreign languages

你会说外语吗?

【误】The fire spread the building quickly but everybody could escape.

【正】The fire spread the building quickly but everybody was able to escape.

大火迅速蔓延到整幢大楼，但大家都逃了出来。

【正】The fire spread the building quickly but everybody managed to escape.

大火迅速蔓延到整幢大楼，但大家都想法逃了出来。

②be able to比can有更多形式。

No one could answer the question.

没人能回答这个问题。(这里could可用was able to代替)

When he grows up, he will be able to support his family.

他长大后就能养家了。

Frank is ill. He hasn't been able to go to school for one week.

弗兰克病了，已经一周没去上学了。

I'm sorry for not being able to help you in time.

对不起，不能及时帮你的忙。

③could经常和动词see, hear, smell, taste, feel, remember, understand等连用。

When we went into the house, we could smell something burning.

当我们走进屋子时，我们闻到什么东西烧焦了。(不用was able to)

She spoke in a very low voice, but I could understand what she said.

虽然她讲话的声音很低，但我还是明白了她说的话。

④在谈论说话时发生的动作，用can，不用be able to。

【误】Look! I'm able to swim.

【正】Look! I can swim.

看，我会游泳了!