I’m puzzled about the usage of a modal verb could. My book “LONGMAN ENGLISH GRAMMAR PRACTICE” by L.G Alexander says that
We use was/were able to or managed to (not could) to describe the successful completion of a SPECIFIC ACTION, for example, We were able to (we managed to) get tickets for the match yesterday (not could)
It was all okay for me till I read another thing about can/could with verbs of perception. There was a sentence which you had to change from
I understood what he said
I could understand what he said.
I thought that correct answer should be
I was able to understand what he said
It’s so confusing, above it says that when we are talking about SPECIFIC ACTION in the past we can’t use could and in the next exercise the key says to go for could.?
“I could get tickets for the match” would be interpreted as meaning “I would be able to, if (something),” for example, if you want me to. “I would be able to get tickets for the match, if you want me to.” But that’s not what we are trying to say. Therefore, we need to use the alternate formulation, “I was able to,” so as to avoid the misunderstanding.
With “I could understand what he said,” we don’t have that problem. This sentence is less likely to be misinterpreted. Let’s say the guy was mumbling and I couldn’t understand him. In other words, if he hadn’t mumbled, things would have gone better. In this case, I would say, “I could have understood what he said if he hadn’t been mumbling.”
“To be able to” will always work. So, if you are in any doubt, that is a safe formulation to use.