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.
We use could to talk about general abilities in the past.
- He could speak fluent French when he was 5.
- I could read before I started school.
Ability on one occasion
However, when we talk about ability on one particular occasion, we can’t use 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.
The burglar could get in through the bathroom window.
Ability on one occasion - negative
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
- 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
Sometimes things were possible in the past but didn’t happen.
- 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.
- 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.