How you can use "could" + infinitive..
1) request permission to do something
Could I ask you a question?
Could I ask a question please?
Could we go home now?
(In this case "could" is the polite version of "can"; "may" is another polite version."
2) past ability
positive form: general ability ( particular ability = managed to / succeeded in)
She could speak several languages.
negative form: general + particular ability
I couldn't read very well when I was young.
Simon was so drunk that he couldn't open the door.
3) make suggestions
We could meet at the weekend.
You could eat out tonight.
4) future possibility
We could go climbing this summer, but I doubt we'll have time.
It could rain later on this evening.
5) asking someone to do something
Could you tell me the time please?
Could you help me with these bags?
6) offer to do something
Could I buy you a drink?