We use can/could to describe ability, and we use can’t/couldn’t to describe absence of ability.
- She could play the piano very well.
- Children can’t concentrate for long periods of time.
- I can speak Chinese.
We also use can to talk about possibility.
- Anyone can win this competition. All the participants are very good.
- I can’t come to the party next week.
We use can/could to talk about something that is or was allowed and we use can’t/couldn’t to talk about something that is not allowed.
- You can’t smoke in the house, but you can smoke in the terrace if you want.
We also use can to ask for permission.
- Can I come in?
We us can/could in requests, i.e. when we ask somebody to do something.
- Can you help me?
- Could you speak more slowly, please?
see, smell, hear, feel, taste
Verbs of perception (see, smell, hear, etc.) are stative verbs and cannot be used in a continuous form. Instead, we use can.
- Can you hear that? (NOT
Are you hearing that?)
- I can smell gas! (NOT
I’m smelling gas!)
be able to
Be able to is often similar to can and we may use it instead of can to talk about ability. But be able to is more formal and is not as common.
- They are able to do the job faster than anyone else.
But can only has present and past forms: can–could. For all other verb forms (infinitive, gerund, present perfect, etc.) we should use be able to.
- I haven’t been able to study for the exam. (present perfect)
- I’d like to be able to talk to her more often. (to-infinitive; NOT
- I don’t like loud music in pubs. I like being able to talk with my friends. (-ing form)
- Will we be able to talk to the singer after the concert? (infinitive)
can/could vs be able to
We can use can and be able to to talk about general ability in the present.
- She can speak five languages.
- She’s able to speak five languages.
We can use could and be able to to talk about general ability in the past.
- My son could walk when he was 11 months old.
- My son was able to walk when he was 11 months old.
But if we want to say that someone did something in a specific situation, we must use was/were able to, or managed to + infinitive.
- There was a fire in the office but all the workers were able to/managed to escape. (NOT
- He lost his maps but after driving around for a long time, he was able to/managed to find the place. (NOT
In negative sentences, both could and be able to can be used to say that we were not capable of doing something on a specific occasion.
- It was very dark and they couldn’t find the exit. (Or They weren’t able to find the exit.)