Modals to express ability: CAN, COULD, BE ABLE TO

Present and future

Can and be able to can express present and future ability:

Tom can play the piano.
We can't enter this street because it's blocked.
I am not able to help you at the moment.
If you ask him, he can probably help you.
I will probably be able to get there by 9.

Past

Could and was/were able to can express general past ability:

I could swim when I was five.
I was able to swim when I was five.

But the past ability to perform a particular action on one occasion is expressed with was/were able to:

Although the current was strong, I was able to swim to the other bank. (I managed to swim to the other bank)

In the sentence above could is not possible. However, this rule is relaxed in the negative and with verbs of the senses:

I read it but I couldn't understand / wasn't able to understand it.
I could see / was able to see him through the window.
I couldn't walk / wasn't able to walk any further.

Be able to is used to express past ability when could is grammatically not possible:

I have been able to swim since I was five.