‘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.