Originally Posted by scheng75
Well I am a native English speaker and I'm not sure I understand the difference (and certainly not the grammar). In UK usage I think the two are virtually interchangeable, but there are subtle differences.
Use of could is more likely when the question is about willingness of the other party to do something, or the possibility of doing it. Use of can is more likely if the question is about their ability to do something.
There is another subtlety in that (as you have suggested) use of could is perhaps a gentler/less direct way of asking the question. Use of can gives a more direct question.
But I think the distinctions are slight.