Learn English – Difference between “couldn’t” and “wasn’t able to”


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?

For example,

  • 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 ?

Best Answer

We use could for general ability. However, when you want to say that somebody did something in a specific situation, use was/ were be able to or managed to (not could).

For example:

There was a boy in the river but people were able to save him.

This is not an ability, but a specific event.

However, negative 'couldn't' is possible in all situations.

I couldn't swim.

We couldn't save the kid.

He played well, but he couldn't beat me.

Lastly, I will give you an example for you to think about it:

Jack was an excellent player. He could beat anybody. (He had the general ability to beat anybody)

Ted and Jack played tennis yesterday. Jack played great but Ted was able to beat him. (Ted, in this specific event, managed to beat Jack)

I can understand the person who suggested 'repeated actions'. When it is a general ability, you will repeat it probably.