COULD vs. WAS/WERE ABLE TO
1. We ONLY use 'was/were able to' if we are talking about what happened in a particular situation or are referring to a specific achievement in the past.
CORRECT: I was able to get a ticket to the concert.
INCORRECT: I could get a ticket to the concert.
Here, getting tickets to a particular concert indicates ability related to a single event (it isn't like the person couldn't get tickets in general. Hence, we ONLY use 'was/were able to' here)
2. However, with stative verbs (verbs of perception such as see, hear; or verbs of thinking such as understand), we can use 'could' as well as 'was/were able to'. Although, 'could' is more common in such cases.
CORRECT: I could see the rainbow from my window.
CORRECT: I was able to see the rainbow from my window.
3. In the negative form, (even if it is a single event), we can use either.
We use wasn't/weren't able to OR couldn't to talk about an negative ability related to a single event.
CORRECT: I wasn't able to get a ticket to the concert.
CORRECT: I couldn't get a ticket to the concert.
Because, the sentence is negative, we can use either of the two.