I don't quite understand a difference between "couldn't" and
"wasn't able to". Someone suggested that we use "wasn't able to" when we talk about one action in the past and "couldn't" when repeated actions in the past. Is that the case?
- I wasn't able to sleep last night
I wasn't able to fall asleep last night
I couldn't sleep last night
- I couldn't fall asleep last night
Which ones are right ?