Expressing ability and inability.
1. We can use can (or to be able to) to describe natural or learned ability.
She can run 1500 metres in 5 minutes.
2. We can use could, couldn’t, was/were able to when we describe ‘general ability in the past’.

I could run very fast when I was a girl.
3. We use was/were able to or managed to (not could) to describe the successful completion of a specific action:

We were able to get plane tickets last week.
4. We can use couldn’t to describe a specific action not successfully completed.

We could not get tickets for the concert last month.
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Verbs of perception with can and could.
1. When we are describing something that is happening now, we do not use the progressive with these verbs.

I see a bird in that tree.
2. We use can + verb in place of the present simple with verbs of perception.

I can see a bird in that tree.
We use could + verb in the place of the simple past with verbs of perception.

I looked up, but couldn’t see anything.
L.G. Alexander. Longman English grammar practice for intermediate students- Harlow: Longman, 296p.