Ability
(Can, could) I help him, please?
Could I help him, please?
(Can, could) I ask you a question, please?
Could I ask you a question, please?
(Can, could) they leave the class early today?
Can they leave the class early today?
We (can, could) go to the cinema once a week when we were teens.
We could go to the cinema once a week when we were teens.
...and visitors (may, could) use car park next to the hotel.
...and visitors may use car park next to the hotel.