These words are used to talk about ability, awareness, and possibility. They are also used to say that someone has permission to do something. These uses are dealt with separately in this entry. Can and could are called modals. Modals
Both can and could are followed by an infinitive without to.
I envy people who can sing.
I could work for twelve hours a day.
The negative form of can is cannot or can't. Cannot is never written ‘can not’. The negative form of could is could not or couldn't. To form the negative of be able to, you either put not or another negative word in front of able, or you use the expression be unable to.
Many elderly people cannot afford telephones.
I can't swim very well.
It was so dark you could not see anything.
They couldn't sleep.
We were not able to give any answers.
We were unable to afford the entrance fee.
ability: the present
Can, could, and be able to are all used to talk about a person's ability to do something. You use can or be able to to talk about ability in the present. Be able to is more formal than can.
You can all read and write.
The animals are able to move around, and they can all lie down.
Lisa nodded, unable to speak.
Could is also used to talk about ability in the present, but it has a special meaning. If you say that someone could do something, you mean that they have the ability to do it, but they don't in fact do it.
We could do much more in this country to educate people.
ability: the past
You use could or a past form of be able to to talk about ability in the past.
He could run faster than anyone else.
A lot of them couldn't read or write.
I wasn't able to answer their questions.
If you say that someone was able to do something, you usually mean that they had the ability to do it and they did it. Could does not have this meaning.
After two weeks in bed, he was able to return to work.
The farmers were able to pay their employees' wages.
If you want to say that someone had the ability to do something but did not in fact do it, you say that they could have done it.
You could have given it all to me.
You could have been a little bit more careful.
If you want to say that someone did not do something because they did not have the ability to do it, you say that they could not have done it.
I couldn't have gone with you, because I was in London at the time.
If you want to say that someone had the ability to do something in the past, although they don't now have this ability, you say that they used to be able to do it.
I used to be able to sleep anywhere.
You used to be able to see the house from here.
ability: the future
You use a future form of be able to to talk about ability in the future.
I shall be able to answer that question tomorrow.
ability: reporting structures
Could is often used in reporting structures. For example, if a woman says ‘I can speak Arabic’, you usually report this as ‘She said she could speak Arabic’.
She said I could bring it back later. Reporting
ability: ‘be able to’ after other verbs
Be able to is sometimes used after modals such as might or should, and after verbs such as want, hope, or expect.
I might be able to help you.
You may be able to get extra money.
You should be able to see that from here.
She would not be able to go out alone.
Do you really expect to be able to do that?
Don't use can or could after any other verbs.
‘being able to’
You can use an -ing form of be able to.
He liked being able to discuss politics with Veronica.
There is no -ing form of can or could.
Can and could are used with verbs such as see, hear, and smell to say that someone is or was aware of something through one of their senses.
I can smell gas.
I can't see her.
I could see a few stars in the sky.
possibility: the present and the future
Could and can are used to talk about possibility in the present or future.
You use could to say that there is a possibility that something is or will be true.
Don't eat it. It could be a toadstool.
He was jailed in February, and could be released next year.
Might and may can be used in a similar way.
It might be a trap.
Kathy's career may be ruined.
Don't use ‘could not’ to say that there is a possibility that something is not true. Instead you use might not or may not.
It might not be possible.
It may not be easy.
If you want to say that it is impossible that something is true, you use cannot or could not.
You cannot possibly know what damage you caused.
It couldn't possibly be true.
You use can to say that something is sometimes possible.
Sudden changes can sometimes have a negative effect.
possibility: the past
You use could have to say that there is a possibility that something was true in the past.
He could have been in the house on his own.
Might have and may have can be used in a similar way.
She might have found the information online.
It may have been a dead bird.
You also use could have to say that there was a possibility of something being true in the past, although it was not in fact true.
It could have been worse.
He could have made a fortune as a lawyer.
Don't use ‘could not have’ to say that there is a possibility that something was not true. Instead you use might not have or may not have.
She might not have known the password.
If you want to say that it is impossible that something was true, you use could not have.
The decision couldn't have been easy.
The man couldn't have seen us at all.
Can and could are used to say that someone is allowed to do something.
You can take out money at any branch of your own bank.
He could come and use my computer.
Cannot and could not are used to say that someone is or was forbidden to do something.
You can't bring strangers in here.
Her dad said she couldn't go out during the week. Permission