So to answer your question, "can" is used to ask for or state facts, while "could" generally is more personal. Usually the context makes it clear.

When use could and can?

Can, like could and would, is used to ask a polite question, but can is only used to ask permission to do or say something ("Can I borrow your car?" "Can I get you something to drink?"). Could is the past tense of can, but it also has uses apart from that--and that is where the confusion lies.

Can you do it or could you do it?

They are both correct the only difference is that saying could is more polite to say than can when asking for a favour. Both of them are good, but 'could' is more polite. Can is more informal, and is 80% strong. Could is 100% strong.