can和could
1.表示能力，可譯為“能，會(huì)”。
I can swim.我會(huì)游泳。
Emily can dance well and her mother could dance well when she was young.
艾米麗舞跳得很好，她媽媽年輕時(shí)舞跳得也很好。
The cinema can seat 1,000 people.
這電影院能容納1，000人。
I could not read such an easy book when I was 7 years old.
我七歲時(shí)連如此簡(jiǎn)單的書我也看不懂。
2.表示允許、許可，常用在口語(yǔ)中。could比can語(yǔ)氣上要客氣。
--- Could I use your dictionary
我可以用一下你的字典嗎?
---Yes, go ahead.
可以，用吧。(或Yes, you can.但不能說Yes, you could.)
Could Can you tell me how to get to the zoo
勞駕，你能告訴我怎么去動(dòng)物園嗎?
He asked me whether he could take the book out of the reading room.
他問我可不可以把書帶出閱覽室。
3.表示推測(cè)，可用于肯定句(can罕見)、否定句和疑問句，但更常見于疑問句或否定句。在此種用法中can和could沒有時(shí)態(tài)的區(qū)別，只是表示可能性的大小，can表示推測(cè)的可能性比could大。
Can he be ill at home
他會(huì)是生病在家嗎?
Can the story be true
這個(gè)故事會(huì)是真的嗎?
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.
在林子里走時(shí)一定不要吸煙，那樣可能會(huì)引起火災(zāi)。
4.表示驚異、懷疑、不相信等態(tài)度，主要用于否定句和疑問句中。
How can you be so careless
你怎么這么粗心?
Where can could they have gone
他們會(huì)去了哪里?
He can'tcouldn't be over sixty.
他不可能超過60歲。
5.Could可以用在虛擬條件句中。
If I could fly, I should be very happy.
如果我能飛，我會(huì)很高興。
If you had followed my advice, you could have finished it.
如果你當(dāng)時(shí)聽我的，你早就做完了。
If you had tried harder, you could have passed your exam.
如果你再努力些，你就能通過考試了。
6.can與be able to的區(qū)別
①can表示“能力”時(shí)，和be able to相當(dāng)，許多場(chǎng)合都可以互相替換。但當(dāng)敘述過去經(jīng)過一番努力才能完成的事情或前面有特殊說明，表示你有能力時(shí)，只能用be able to。
【正】Can you speak any foreign languages
你會(huì)說外語(yǔ)嗎?
【正】Are you able to speak any foreign languages
你會(huì)說外語(yǔ)嗎?
【誤】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樓，但大家都逃了出來。
【正】The fire spread the building quickly but everybody managed to escape.
大火迅速蔓延到整幢大樓，但大家都想法逃了出來。
②be able to比can有更多形式。
No one could answer the question.
沒人能回答這個(gè)問題。(這里could可用was able to代替)
When he grows up, he will be able to support his family.
他長(zhǎng)大后就能養(yǎng)家了。
Frank is ill. He hasn't been able to go to school for one week.
弗蘭克病了，已經(jīng)一周沒去上學(xué)了。
I'm sorry for not being able to help you in time.
對(duì)不起，不能及時(shí)幫你的忙。
③could經(jīng)常和動(dòng)詞see, hear, smell, taste, feel, remember, understand等連用。
When we went into the house, we could smell something burning.
當(dāng)我們走進(jìn)屋子時(shí)，我們聞到什么東西燒焦了。(不用was able to)
She spoke in a very low voice, but I could understand what she said.
雖然她講話的聲音很低，但我還是明白了她說的話。
④在談?wù)撜f話時(shí)發(fā)生的動(dòng)作，用can，不用be able to。
【誤】Look! I'm able to swim.
【正】Look! I can swim.
看，我會(huì)游泳了!
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