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.
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.