'Could' can be used to talk about the past, the present or the future.
'Could' is a past form of 'can'
'Could' is used to make polite requests. We can also use 'can' for these but 'could' is more polite.
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'
'Could' is used to talk about theoretical possibility and is similar in meaning to 'might'.
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