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。
