Most of the people says I wish I could, I wish you would.
Can't we use I wish I can, I wish you will?
I'd like to know what the main difference between the usage of can/will and could/would is.
In this context, could and would are used to express an hypothetical scenario. Can and will in the contrary have a sense of immediacy and certainty, so it wouldn't fit.
"Can" asks the possibility, as in "Can you help me?". "Would" asks the person addressed if they have the will, as in "Would you help me?"
So "would" or "will" is more polite, implying that the person addressed has a choice to act.