We use can to say that someone has the ability or opportunity to do something. The negative of can is cannot (contraction: can't).

• Can you swim?
• He can play the guitar.
• It's nice today. We can sit in the garden.
• I can't open this bottle.

Can usually expresses the idea that something is pos­sible because certain characteristics or conditions exist. Can combines the ideas of possibility and ability.

• Tom is strong. He can lift that heavy box. (Itis possible for Tom to lift that box becausehe is strong.)
• I can play the piano. I've taken lessons for many years. (It is possible for me to play the piano because I have acquired that ability.)
• That race car can go very fast. (It is possible for that car to go fast because of its special characteristics.)
• Can you meet me tomorrow evening? (Is it possible for you to meet me? Will you be free?)
• We can use be able to instead of can eg Are you able to swim? but can is more common.

Could and was/were able to

We can use could to say that someone had the general ability to do something in the past.

• I could swim when I was 4 years old.
• My sister could talk when she was 15 months old.

We also use was/were able to with this meaning.

• I was able to swim when I was 4 years old.

But when we want to say that someone had the ability to do something, and that they did it in a particular situation, we must use was/were able to (could is not possible).

• Even though I'd hurt my leg, I was able to swim back to the boat. (Not: ... I could swim back ...)
• The manager wasn't in the office for very long, but we were able to speak to him for a few minutes. (Not: we could ayeak to him ...)

We can use managed to (+ infinitive) or succeeded in (+ -ing form) instead of was/were able to in this meaning.

• Even though I'd hurt my leg, I managed to swim back to the boat/I succeeded in swim­ming back to the boat.

We normally use managed to or succeeded in when the action was difficult to do.

There is an exception with the verbs of perception see, hear, smell, taste, feel, and some verbs of thinking eg understand, remember. We use could

with these verbs when we actually did these things in particular situations.

• We could see a man in the garden.
• I could hear a noise outside my bedroom door.

We use could not (contraction: couldn't) for both general ability and particular situations.

• My grandmother couldn't dance.
• He tried very hard, but he couldn't swim back to the boat.

Could have ...

We use could have + past participle to say that someone had the ability or the opportunity to do something in the past but did not do it.

• You could have helped me. Why didn't you? I could have gone to China on holiday last year, but I decided not to.

Expressing ability in other forms: be able to

Can has no infinitive, -ing form or participles. So, when necessary, we make these forms with be able to.

• I'd like to be able to play the piano. (We cannot say I'd like to can play ... )
• In the future, people will be able to live on other planets. (We cannot say ... people will can live ... )
• She enjoys being able to speak foreign langua­ges. (We cannot say Ghe enjoys canning ...)
• I've been able to drive since I was 18. (We cannot say I've could ...)

Possibility: can

We use can to talk about 'theoretical possibility'.

• You can ski on the hills. (= It is possible to ski, i.e. circumstances permit: there is enough snow. )
• Anyone can learn to swim. (= It is possible for anyone to learn to swim.)
• We can't bathe here on account of the sharks. (It isn't safe.)
• Can you get to the top of the mountain in one day? (Is it possible?)

In this use,  can often has a similar meaning to 'sometimes'.

• My brother can be very nice. (= My brother is sometimes very nice.)
• The Straits of Dover can be very rough. (= It is possible for the Straits to be rough; this sometimes happens.)

We use could to talk about theoretical possibility in the past.

• My brother could be really horrible when he was a child.