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Ability: can and could

Knowledge, skill, strength, etc: I can read Italian.​

We use can to say what people and things are able (or unable) to do because of their knowledge, skill, strength, nature, design, etc.

  • I can read Italian, but I can’t speak it.
  • Dogs can’t climb trees.
  • Henry can lift 100 kilos.
  • These roses can grow anywhere.
  • Can gases freeze?
  • My car can do 150mph.

Be able to (see here) is used with similar meanings, especially when we are talking about people’s ability.

  • Henry is able to lift 100 kilos.

Possible in the situation: We can go to Paris.​

We also use can to say what we are able (or unable) to do because of the circumstances that we are in – what is possible in the situation.

  • We can go to Paris this weekend, because I don’t have to work.
  • I can’t come out this evening: I have to see my brother.
  • There are three possibilities: we can go to the police, we can talk to a lawyer, or we can forget all about it.
  • ‘What can we do?’ ‘We can try asking Lucy for help.’
  • Anybody who wants to can join the club.

Past: She could read when she was four.​

We use could to talk about general ability in the past.

  • She could read when she was four.
  • My grandmother could sing like an angel.
  • My last car could do 160mph.
  • In those days everybody could find a job.
  • It could be quite frightening if you were alone in our big old house.

Was/were able to is also possible, especially to talk about people’s ability.

  • She was able to read when she was four.

Past: could is not always possible​

We use could for ‘general ability’ – for example to say that somebody could do something at any time, whenever he/she wanted.

  • When I was younger, I could run 10km in under 40 minutes.

We do not normally use could to say that somebody did something on one occasion. Instead, we use other expressions.

  • I managed to run 10km yesterday in under an hour. (not I could run 10km yesterday …)
  • How many eggs were you able to get? (not … could you get?)
  • After six hours’ climbing, we succeeded in getting to the top of the mountain. (not … we could get to the top …)
  • I found a really nice dress in the sale. (not I could find …)

However, we use couldn’t to say that something did not happen on one occasion.

  • I managed to find the street, but I couldn’t find her house.

Other uses of could​

Could is not only past: we also use it as a ‘softer’, less definite form of can.

  • ‘What shall we do tomorrow?’ ‘Well, we could go fishing.’
  • When you’re in Spain, you could go and see Alex.

Could can mean ‘would be able to’.

  • You could get a better job if you spoke a foreign language.

Could is used in past indirect speech, when can was used in direct speech.

  • ‘Can you help me?’ ‘What did you say?’ ‘I asked if you could help me.’

could have …​

We use a special structure to talk about unrealised past ability or opportunities – to say that somebody was able to do something, but did not do it; or that something was possible, but did not happen.

Grammar

could have + past participle

  • I could have married anybody I wanted to.
  • I was so angry I could have killed her!
  • Why did you jump out of the window? You could have hurt yourself.
  • I could have won the race if I hadn’t fallen.

This structure can be used to criticise people for not doing things.

  • You could have helped me – why did you just sit and watch?

Negative sentences suggest that somebody would not have been able to do something even if they had wanted or tried to.

  • I couldn’t have won, so I didn’t go in for the race.
  • I couldn’t have enjoyed myself more – it was a perfect day.

The structure is sometimes used to talk about past events which are not certain to have happened (like may/might have – (see here).

  • ‘Who sent those flowers?’ ‘I’m not sure. It could have been your mother.’

This structure can also refer to present situations which were possible but which have not been realised.

  • He could have been Prime Minister now if he hadn’t decided to leave politics.
  • We could have spent today at the beach, but we thought it was going to rain so we decided not to go.

Chances: Will it happen? / Is it happening? – can not used​

We do not use can to talk about the chances (probability) that something will actually happen, or is actually happening. Instead, we use may (see here).

  • We may go camping this summer. (not We can go …)
  • ‘Where’s Sarah?’ ‘She may be with Joe.’ (not She can be …)

However, could is possible in this sense.

  • It could rain later this evening, perhaps. (but not It can rain later …)