Ability
We can use can/could to express general ability and typical behaviour of people or things
– Temperatures can rise to over 30°C in the summer.
– Employers can be unwilling to employ people over 50.
– My father could be very generous. (past)
We use can/be able to for present and future ability
– I can use a word processor but I can’t type very well.
– Will your parents be able to help you?
– I like being able to cook my own meals.
We use can for the future where there is a sense of opportunity
– I can come tomorrow if you like.
– You can practise your French when you go to Paris.
We use could/couldn’t to talk about general past ability
– I could swim before I could walk.
– Andrew’s father couldn’t get a job.
We use was/wasn’t able to talk about past ability in a specific situation
– Fortunately, he was able to swim to the shore.
We use could/couldn’t + perfect infinitive to talk about unfulfilled ability in the past
– I could have gone to university, but I decided not to.
– I couldn’t have been a ballet dancer. I was too tall.
Other expressions for ability
– Do you know how to type?
– He succeded in becoming a professional footballer at 18.
– We managed to find our way home. (suggests difficulty)
Possibility
We use can or could for theoretical possibility
– Can there be life on Mars?
– Can that be Peter over there?
– The weather could be better tommorow. (it’s possible)
We use may, might, could + infinitive to talk about likelihood in the present or future
– He may be in a meeting. (quite likely to be true)
– She might/could be here already. (less likely)
We use could/may/might + have + past participle (perfect infinitive) to talk about the possibility that past events happened
– His face was familiar. We may have met somewhere before.
– He’s not in the office. He might have finished work early.
– She could have been at the party, but I didn’t see her.