'Could' can be used to talk about the past, the present or the future.
'Could' is a past form of 'can'
- When I was living in Boston, I could walk to work.
- He phoned to say he couldn't come.
- I could see him clearly but I couldn't hear him and then the videoconference line went dead.
'Could' is used to make polite requests. We can also use 'can' for these but 'could' is more polite.
- Could you help me, please?
- Could you lend me some money?
- Could I have a lift?
- Could I bother you for a moment?
If we use 'could' in reply to these requests, it suggests that we do not really want to do it. If you agree to the request, it is better to say 'can'.
- Of course I can.
- I could help you if it's really necessary but I'm really busy right now.
- I could lend you some money but I'd need it back tomorrow without fail.
- I could give you a lift as far as Birmingham.
'Could' is used to talk about theoretical possibility and is similar in meaning to 'might'.
- It could rain later. Take an umbrella.
- He could be there by now.
- Could he be any happier?
- It could be Sarah's.