‘Can’ and ‘could’ are modal verbs used in a few different ways.

Use ‘can’ to talk about:

Ability:

I can swim well.

Possibility:

When mum gets back from shopping we can go out.

Opportunity:

I am not working today so we can go to the beach.

Permission:

You can park here.

Use ‘could’ to talk about:

Possibility in the past:

Last year, we could go on holiday wherever we wanted.

Uncertain possibility in the future:

She could fail her exam unless she studies more.

Use either ‘can’ or ‘could’ to ASK permission. ‘Could’ is more polite and formal:

Can I have another cup of coffee?

Could I borrow your car?

However, to GIVE permission use ‘can’ or ‘cannot’.

You can go out now.

You can’t borrow my car.