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
to
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?