We use the modal can to make general statements about what is possible:
It can be very cold in winter. (= It is sometimes very cold in winter)
You can easily lose your way in the dark. (= People often lose their way in the dark)
We use could as the past tense of can:
It could be very cold in winter. (=Sometimes it was very cold in winter.)
You could lose your way in the dark. (=People often lost their way in the dark)
We use could to show that something is possible in the future, but not certain:
If we don’t hurry we could be late. (=Perhaps/Maybe we will be late)
We use could have to show that something is/was possible now or at some time in the past:
It’s ten o’clock. They could have arrived now.
They could have arrived hours ago.
We use the negative can’t or cannot to show that something is impossible:
That can’t be true.
You cannot be serious.
We use couldn’t/could not to talk about the past:
We knew it could not be true.
He was obviously joking. He could not be serious.
We use can to talk about someone’s skill or general abilities:
She can speak several languages.
He can swim like a fish.
They can’t dance very well.
We use can to talk about the ability to do something at a given time in the present or future:
You can make a lot of money if you are lucky.
Help. I can’t breathe.
They can run but they can’t hide.
We use could to talk about past time:
She could speak several languages.
They couldn’t dance very well.
We use can to ask for permission to do something:
Can I ask a question, please?
Can we go home now?
could is more formal and polite than can:
Could I ask a question please?
Could we go home now?
We use can to give permission:
You can go home now if you like.
You can borrow my pen if you like.
We use can to say that someone has permission to do something:
We can go out whenever we want.
Students can travel free.
Instructions and requests:
We use could you and as a polite way of telling or asking someone to do something:
Could you take a message please?
Could I have my bill please?
can is less polite:
Can you take a message please?
Offers and invitations:
We use can I … to make offers:
Can I help you?
Can I do that for you?
We sometimes say I can ... or I could ... to make an offer:
I can do that for you if you like.
I can give you a lift to the station.