Can fits present ability and general possibility; could fits past reference, polite requests, or softer possibility. In a message, the difference shows in tone:
Could we start Monday morning?
Can we start Monday morning?
Both are correct; could just sounds softer with someone new.
Same message, two tones
Could we start Monday morning? What time works for you? You are writing to a new client. The distance reads as politeness. Can we start Monday morning? What time works for you? Same request, more direct, fine with a teammate you message daily.
You could try the north entrance. In many contexts this is heard as a gentle suggestion. You can try the north entrance. This states an option that exists. Both are correct, so your intent decides: recommending or informing.
I could finish it tonight, if you need it early. A real possibility, stated with some uncertainty. I can finish it tonight. A firmer offer of ability. Pick the version that matches how sure you are.
In a message you are composing now, the choice is mostly tone. Can we start Monday morning? is direct. Could we start Monday morning? asks the same thing with a little distance, and that distance reads as politeness.
Could we start Monday morning? What time works for you?
Can we start Monday morning? What time works for you?
Both are correct. Could suits a new client, where the distance feels courteous. Can suits a teammate you message daily, where it sounds brisk and confident. Your relationship with the reader decides, not the grammar.
Could is historically the past of can, so when your sentence points backward, could reports past ability: I could read by five. In a forward-looking request, it is not about the past at all. It borrows that distance to soften the ask.