情態動詞can與could的用法歸納

1. 表示能力

(1) 表示現在的能力，用can，如：

He can (=He is able to) swim. 他會游泳。

He can speak English well. 他英語能說得很好。

I can’t remember where I put it. 我記不得把它放在那兒了。

(2) 表示將來的能力，通常用will / shall be able to，如：

I shall be able to come tomorrow. 明天我能來。

One day people will be able to go to the moon on holiday. 總有一天人們可以到月球上去度假。

但是，若表示現在決定將來是否有能力做某事，則可用 can，如：

Can you come to the party tonight？你今晚能來參加我們的聚會嗎？

(3) 表示過去的能力。

① 若泛指過去的能力，可用could，如：

I could run faster then. 我那時能跑得更快一些。

②若表示過去特定場合做某事的能力，則不能用could，而用was / were able to do sth.，或用 managed to do sth，或用 succeeded in doing sth 等。如：

He studied hard and was able to pass the exam. 他學習很努力，所以考試能及格。

= He studied hard and managed to pass the exam.

= He studied hard and succeeded in passing the exam.

誤：She could jump into the sea before the ship was blown up. 船爆炸之前，她跳進了大海。

正：She was able to jump into the sea before the ship was blown up.

注：在下列情況，could也可表示過去特定的能力：

①在否定句中，could和was / were able to可通用。如：

I couldn’t / wasn’t able to see him before he left. 他離開之前我沒有再見到他。

②could與see, hear, smell, taste, feel, understand等表示感知的動詞連用時。如：

I can hear you easily from here. 我在這兒就能很輕易地聽到你的話。

Then he spoke slowly, but I could not understand him. 接著他說得慢了,但是我聽不懂他的話。

③could用在間接引語中時。如：

He said he could finish the work next week. 他說他下周能完成工作。

順便說一下，be able to還可用于完成時, 如：

I’m sorry I haven’t been able to come earlier. 對不起，我沒有能夠來得更早一些。

He said he had not been able to answer your letter. 他說他一直沒能給你回信。

2. 表示許可

(1) 對于現在或將來的許可，用can或could均可，但could 語氣更委婉, 如：

Can / Could I smoke in here? 我可以在這兒吸煙嗎?

You can't pick the ball up in football. 踢足球時不許可用手持球。

I could come tomorrow if you like. 如果你愿意，明天我可以來。

② 表示自己允許別人做某事時, 答語應用can或may，而不用 could。如：

--Could / Can I use your pen? 我可以借用你的鋼筆嗎?

誤：-- Yes, of course you could. 當然可以。

正：--Yes, of course you can / may.

(2) 對于過去的許可。 若表示過去一般性允許用could；表示過去特定的允許，則不用 could，而需換成其它表達，如had permission, was / were allowed to。如：

When I lived at home, I could watch TV whenever I wanted to. 我住在家里時, 想什么時候看電視就什么時候看。(一般性允許)

I was allowed to see the film yesterday evening. 昨天晚上允許我去看了電影。(特定的允許)

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