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。