Can and Could

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Possibility 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.

Impossibility: 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.

Ability: 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.

Permission: 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.

Source:

https://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs/can-or-could

http://youtu.be/NxgDEz07Jvg (Embed)

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