三、can/could have done的用法

1.can/could have done表示“對過去的可能性推測”，could的語氣較can弱。

該結構表推測時無肯定式，一般用于否定句和疑問句。否定句表示“過去不可能發生過某事”，它是must have done的否定式；疑問句表示“過去可能發生過某事嗎”。例如：

    He can’t have forgotten it.他不可能已忘記此事。

    Can he have gone to his aunt's?他有可能到他姑姑家去了嗎？

I saw Mr. Zhang just now and he couldn't have gone to Beijing.我剛才看見張先生了，他不可能去了北京。

Why does he know this? Could someone have told him about it? 他怎么知道? 會是哪個人告訴他了嗎?

We could not have heard them because of the noise from the river. 由于有河水聲我們不能聽見他們的話。

Can he have left already? 他會已經離開了嗎?

They can’t have gone out because the light’s on. 他們不可能出去了，因為燈還亮著。

注意：在非正式英語或口語中，偶見到could have done也可用于肯定句，表示推測：

She could have forgotten to tell him. 她可能忘記告訴他了。

Tom could have taken the money; he was here alone yesterday. 有可能是湯姆把錢拿走了，昨天他單獨一個人在這里呆過。

In the example mentioned above, either method of construction could have been used.在上面提到的例子中，兩種建造方法均可能用過。

2. could have done 表示未曾實現的可能性

該結構僅限于使用could，不用can。表示過去本來有能力、有可能、有打算、有選擇做某事的，但實際上卻沒有那樣做。即“過去本來可能做某事，而實際沒有做”。通常譯為“本來可以”，“本來能夠”等。例如：

You could have done better, but you didn't try your best.你本可能做得更好，但你沒有盡力。

We could have got in for nothing—nobody was collecting tickets. 我們本可以不買票就進去——根本沒人收票。

I could have passed my examination easily but I made too many stupid mistakes. 我本可以輕易通過考試，但我犯了太多不該犯的錯誤。

You could have stayed with me when you were in this city on business.在這個城市出差時你本能夠和我住在一起的。

She could have been seriously injured. 她本來可能傷得很重的。

It’s difficult enough, but it could have been worse. 這事情夠困難的了，本來可能會更糟呢！

I could have given you some hints, but I suppose you thought yourself too grand. 我本可以給你點提示的，但是我認為你太自負了。

I could have lent you the money. Why didn’t you ask me? 我本來可以借這筆錢給你的，你為什么不向我提出?

You needn’t have walked up; you could have taken the lift. 你沒必要走著上去，你本來可以乘電梯的。

有時，也可根據語境譯為“好在沒有”，“差點”，尤其是當談論的是不利情況時。如：

She felt miserable. She could have cried. 她感到很痛苦，她差點哭了。

That was a had place to go skiing—you could have broken your leg. 那不是一個滑雪的好地方，好在你沒有把腿給摔斷。

Why did you throw the bottle out of the window? Somebody could have been hurt. 你為什么把瓶子扔出窗外呢? 好在沒有砸傷人。

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