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.
在現(xiàn)在時(shí)態(tài)里，我們用can去談?wù)撘话愕哪芰?，例如I can swim。還有形容在一個(gè)特定的時(shí)間的一種能力，例如I can see you！但是，關(guān)于談?wù)撨^(guò)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.
我們用could談?wù)撨^(guò)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
某個(gè)場(chǎng)合的能力

However, when we talk about ability on one particular occasion, we can’t use could
但是，當(dāng)我們談?wù)撃硞€(gè)特定場(chǎng)合下的能力，我們不能用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.
我們必須用was able to 或者managed to，我們不能用could。

? The burglar could get in through the bathroom window.

Ability on one occasion – negative
某個(gè)場(chǎng)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
當(dāng)我們談?wù)撃硞€(gè)特定場(chǎng)合不能做到某事的能力時(shí)，我們用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
假設(shè)的過(guò)去的能力

Sometimes things were possible in the past but didn’t happen.
有時(shí)候，我們要表達(dá)在過(guò)去可能會(huì)發(fā)生但是沒(méi)有發(fā)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.
我們用could have (+ 過(guò)去分詞) or would have been able to 來(lái)談?wù)撨@些假設(shè)會(huì)發(fā)生的事件。他們可以是肯定式，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.