logo  ELT Concourse for learners of English
Concourse 2

Using could and was able to to talk about ability

read Task 2: Now look at this.
He has worked hard so he could pass the examination. This refers to the future - the speaker is saying that it is possible for him to pass.  We are talking about possibility not ability.
He worked hard so he was able to pass the examination. This means that the speaker knows that he passed the examination because he had worked hard and had the ability to pass.
When I was young I could swim well. The speaker is simply saying he/she had the ability.
He could change it. The speaker is asking about a possibility in the future, not something that happened.
Was he able to change it? The speaker is asking about an action in the past.

Click here to go on.