注意了!could 虽然可以表示能力,但它不能表示所有的能力

Could 是常见的情态动词之一,用来表示可能性或过去的能力,以及提出建议和要求等。虽然它可以表示能力,但是它不能表示所有的能力。

Could 用来表示过去的一般的能力(general ability),例如:
I could run ten miles in my twenties.
我二十岁能跑十英里。
I could speak Chinese when I was a kid.
我小时候会说中文。
When I was young, I could easily touch my toes.
当我年轻的时候,我可以很容易地触摸我的脚趾。

但是某些能力在特定情况下只发生一次或一定次数,此时不能用 could 来表示,例如:
昨天,我可以自己移开沙发。
Yesterday, I could lift the couch by myself. (错误)
解析:能移开沙发属于一次性的能力,不属于一般能力的范畴,所以不用 could.
我朋友的电话昨天一整天都占线。最后到了深夜,我可以和他说话了。
My friend’s phone was engaged all day yesterday. Finally, late in the evening, I could speak to him. (错误)
解析:能打电话属于一次性的能力,所以不用 could.

那么如何表达过去的非一般的能力呢?我们可以用 be able to,managed to 或 succeeded in 等,此时它们可以互换。以上的例句对应地可以改为:
1. be able to:
Yesterday, I was able to lift the couch by myself.
My friend’s phone was engaged all day yesterday. Finally, late in the evening, I was able to speak to him.
2. managed to:
Yesterday, I managed to lift the couch by myself.
My friend’s phone was engaged all day yesterday. Finally, late in the evening, I managed to speak to him.
3. succeeded in:
Yesterday, I succeeded in lifting the couch by myself.
My friend’s phone was engaged all day yesterday. Finally, late in the evening, I succeeded in speaking to him.

但是英语语法总是有一些奇怪的现象或例外。虽然我们不能在肯定句中用 could 表示非一般的能力,但是 couldn't 可以用于所有的能力,包括非一般的能力,例如:
When Jane was very young, she couldn’t speak German.
简很小的时候,她不会说德语。
She said that she couldn't come.
她说她来不了啦。
I couldn't hear what they were saying.
我听不见他们在说些什么。
I couldn't get the car to start this morning.
我今天早上没法让这汽车发动起来。
解析:以上两句中的 come 和 hear 都是一次性的能力,此时都可以用 couldn't。

下面看一些常见的错误地在肯定句中用 could 来表示非一般的能力的例句,例如:
I spent an hour looking for my keys and could find them.
我花了一个小时找钥匙,终于找到了。
I could buy the notebook at a good price.
我可以以很好的价格买到这本笔记本。
There was a fire in the building but everybody could escape.
大楼里发生了火灾,但每个人都能逃脱。
I played well for a change and could beat him.
换个角度来说,我打得很好,可以打败他。
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