Study the information about modal verbs ‘can’ and ‘could’.
Modal verb ‘can’:
1. 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)
2. We use could as the past tense of can:
It could be very cold in winter. (=Sometimes it was very cold in winter.)
3. We use could to show that something is possible in the future, but not certain:
If we don’t hurry we could be late.
4. 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.
5. Impossibility:

We use the negative can’t or cannot to show that something is impossible:
That can’t be true.
Mark cannot be serious.
6. We use couldn’t/could not to talk about the past:
They knew it could not be true.
7. Ability:

We use can to talk about someone’s skill or general abilities:
She 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.

We use could to talk about past time:
She could play volleyball.
8. Permission:

We use can to ask for permission to do something:
Can I ask a question, please?
Could is more formal and polite than can:
Could I ask a question please?
We use can to give permission:
You can go home now if you like.
We use can to say that someone has permission to do something:
Students can travel free.
9. Instructions and requests:

We use could you as a polite way of telling or asking someone to do something:
Could you take a message please?
Can is less polite:
Can you take a message please?
10. Offers and invitations:

We use can I … to make offers:
Can I help you?