Modal verbs

Ability

Can, could, be able to, capable of, know how to all express ability.

Can and be able to are similar in meaning. Be able to suggests that effort is required.

• Chimpanzees can count to 10.
• Not many people are able to run a marathon.

Can has only present and past forms.

• Can you play the guitar?
• I could swim faster when I was young.

Be able to is used for other aspects.

• Will you be able to play on Saturday?
• He's never been able to finish a novel.
• If you hadn't helped me, I wouldn't have been able to pass the exam.

Be capable of is often used for things that are difficult or unusual.

• Kevin is capable of memorizing hundreds of phone numbers.
• I can't believe a child would be capable of murder.

Know how to means to be able to do something practical because you have learned it.

• Do you know how to play chess?

• She could read and write by the time she was three.

Was/were able to often combines the idea of ability and achievement.

• The car broke down, but we were able to fix it.

Note that could is not possible in this context.

• The car broke down, but we could fix it.

Managed to is similar in meaning to was/were able to. It suggests that effort was rewarded by success.

• Fortunately, we managed to find a hotel with a room.