Grammar: can/could/be able to

talk about ability: I can ride a bike.

ask for and give permission: Can I go home because I don't feel well?

talk about what is possible: You can keep fit by walking every day.

Ask somebody to do something for you: Can you go to the chemist's?

We use be able to to talk about ability.

We can use it in all tenses except for continuous tenses.

We use a bare infinitive after be able to.

Harry is able to run faster than Jim.

You will be able to get out of bed in two days.

They have been able to get fit with judo.

We use was/were able to to talk about what someone managed to do on a specific occasion in the past: He didn't feel well, but he was able to eat dinner.

Could is usually used only for general ability in the past: I could read when I was 5.

The negative forms couldn't and wasn't /weren't able to can be used for both general and specific ability in the past: The boy couldn't / wasn't able to walk after the accident.