At a café counter with a stranger, use could for your polite request: Could I order a flat white? It still means now, not the past.
With a mate, switch to casual can: Can I try your coffee? Quick check, stranger or friend? Stranger means could.
You can't hand someone permission with could. Asking is fine, giving isn't: Can you use my laptop? grants it, while Could you use my laptop? sounds like you're asking them instead. Keep could for questions, and give permission with can.
Permission you're granting, casually: Sure, you can park in my driveway. With someone you don't know well, the same sentence still works; granting permission doesn't need the softening that asking for it does.
Ability right now: I can see the menu from here. In a question, put the modal before the subject: Can you read that price from where you're standing?
Possibility right now, strength decides: can when you'd rely on it, could when it's only a chance. It can get busy here on Saturdays states something dependable; There could be a queue right now keeps it open.
Future possibility is separate: could alone, because it marks a chance rather than a certainty. It could rain later stays open; It can rain later claims you're sure it will.
Watch a common mix-up: could doesn't suit general present truths. It can take ten minutes at this counter is right; It could take ten minutes makes it sound like a one-off chance instead of the usual run of things.