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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 ?

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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.

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  • Thank you for your reply ! I've meant a bit another thing. Is that usual for people to talk about some event in the past in negotive form using "wasn't able to" ? I mean it seems to me I always face when people say "coudn't" and yesterday I faced that "wasn't able to" also can be used but how often or even usual people use that phrase. Is there the same difference between "couldn't" and "wasn't/weren't able to" like between "could" and "be able to" ? Unfortunetly, my expirience is not so vast like you – Vadim Nov 19 '15 at 9:15
  • Since negative form 'couldn't' can cover all situations (specific or general) you might hear it more. Couldn't can be used instead of wasn't able to. When you hear people saying wasn't able to, I am sure they are talking about something specific they couldn't/ wasn't able to do. Like, I wasn't able to do my homework yesterday. This is not an ability but a specific event. Both expressions are common but as I said, since couldn't can be used instead of wasn't able to, there is a chance you might hear it more – Grizzly Nov 19 '15 at 23:07
  • Ah, I see. It seems like "didn't managed to" and "failed to", right ? – Vadim Nov 20 '15 at 7:16

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