Can, could, and be able to

 

Could and was able to

Sometimes could is the past of can. We use could especially with these verbs.

  see   hear    smell   taste    feel    remember    understand

• When we went into the house, we could smell something burning.
• She spoke in a low voice, but I could understand what she was saying.

We also use could to say that someone had the general ability to do something.

• My grandfather could speak five languages.
• When Joe was 16, he could run 100 meters in 11 seconds.

But if you mean that someone managed to do something in one particular situation, you have to use was/were able to (not could).

• The fire spread through the building very quickly, but everyone was able (= managed) to escape. (not could escape)
• They didn't want to come with us at first, but finally we were able (= managed) to persuade them. (not could persuade)

Compare could and was able to in this example.

• Jack was an excellent tennis player. He could beat anybody. (= He had the ability to beat anybody.)
• But once he had a difficult game against bob. Bob played very well, but in the end Jack was able to beat him. (= He managed to beat him in this particular game.)

The negative couldn't is possible in all situations.

• My grandfather couldn't swim.
• We tried hard but we couldn't persuade them to come with us.