From what I've read and learned in lessons, 이/가 are general subject markers, whereas 은/는 kind of convey a subtle "as opposed to" feel. Ex: 고양이가 집에 있어요. = The cat is in the house. 고양이는 집에 있어요. = The CAT is in the house. (In context maybe it would be something like: The dog is outside, whereas the cat is in the house). But that's just my very limited, non-native understanding =)

이/가 it's used to emphasize next words 은/는 it's used to emphasize the subject Example: 어머니는 시장에 갔습니다. (my MOTHER went to the market) 어머니가 시장에 갔습니다. (my mother went to the MARKET)

Hei :) I know nothing about korean but this is all I can give you ;) http://forum.wordreference.com/showthread.php?t=1775129