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.
She is able to run faster than him.
You will be able to get out of bed in two days.
They have been able to get fit with yoga.
We use was/were able to to talk about what somone managed to do on specific occasion in the past. Could is usually used only for general ability in the past.
He didn't feel well,but he was able to eat dinner.
Were they able to get to the hospital in time?