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In the present tense we use ‘can’ to talk about a general ability – e.g. I can swim – and also for ability at a particular moment – e.g. I can see you! The rules for talking about past abilities are different.
在现在时态里，我们用can去谈论一般的能力，例如I can swim。还有形容在一个特定的时间的一种能力，例如I can see you！但是，关于谈论过去的能力是不同的。
General ability 一般的能力
We use could to talk about general abilities in the past.
我们用could谈论过去一般的能力。
• He could speak fluent French when he was 5.
• I could read before I started school.
Ability on one occasion 某个场合的能力
However, when we talk about ability on one particular occasion, we can’t use could但是，当我们谈论某个特定场合下的能力，我们不能用could。
• The burglar was able to get in through the bathroom window.
• The burglar managed to get in through the bathroom window.
We have to use was able to or managed to. We can’t use could.
我们必须用was able to 或者managed to，我们不能用could。
• The burglar could get in through the bathroom window.
Ability on one occasion – negative 某个场合的能力—否定式
When we talk about a particular occasion when something wasn’t possible, we can use wasn’t/weren’t able to, didn’t manage to, or couldn’t
当我们谈论某个特定场合不能做到某事的能力时，我们用wasn’t/weren’t able to, didn’t manage to, 或者是couldn’t
• The burglar wasn’t able to get in through the window.
• The burglar didn’t manage to get in through the window.
• The burglar couldn’t get in through the window.
Hypothetical past ability 假设的过去的能力
Sometimes things were possible in the past but didn’t happen.
有时候，我们要表达在过去可能会发生但是没有发生的事情。
• I could have gone to university but I decided to get a job.
• I would have been able to win the race but I fell over.
We use could have (+ past participle) or would have been able to to talk about these hypothetical events. They can be used in the positive and the negative.
我们用could have (+ 过去分词) or would have been able to 来谈论这些假设会发生的事件。他们可以是肯定式，也可以是否定式。
• I couldn’t have done it without your help.
• I wouldn’t have been able to afford it even if it had been for sale.
（来源：英国使馆文化教育处）