could and can

I have learned that in a conditional sentence in the present, we use "simple past" for the If clause.

I don't know if this is calld a conditional sentence, but we say;
"It would be great if you could ..."
"I would appreciate it if you could ..."
I would understand this.

But I think I have also read/heard;
"It would be great if you can ..."
"I would appreciate it if you can ..."
To me, they look gramatically incorrect. But are these OK?

They'd pass in an informal situation but you'll be marked down if you write the second version on a grammar test.

Jim is 100% correct.

However, I subconciously think of the "can" sentences as more of a demand, and the "could" sentence as a request.

If my boss said,
"I would like you to come over sometime this week, if you could."
I'd probably say OK and not think too much more about it. I'd
visit him when I wasn't busy.

If he said,
"I would like you to come over sometime this week, if you can."
I'd probably rearrange my schedule so I could meet with him.
It sounds more like he is saying, "You will meet with me this week,"
just more politely.

Might just be part of my dialect though.