Well, 가 is for only subject but 는 also used with objects, adverbs, etc When we refer to new inoformation, we say 가 whereas 이 is used when we speak about what we already know, or when the first information is involved 차사고가(not 는) 났어요, 그래서 저 도로는(not 가) 이용할 수 없어요 There has been a car accident, so you can't use that road. 그 차사고는(not 가) 매우 위험했어요. The accident was very dangerous. And 는 implies contrast 그는(not 가) 키가 커요 하지만 그녀는(not 가) 작아요 He is tall, but she isn't Yeah,, it's quite confusing to use them for non-native speakers, you should be exposed to them much, and practise :P

another tough Qeustion here again ^^ there is a "조사" in korean... Noun +'조사' is grammer usage. making subject is "Noun + 이/가", making object is "Noun + 을/를/" and there is "보조사" "보조사" is assistance of "조사" "은/는" is "보조사" "이/가" is "주격조사"--> making subject "은/는" is located first place of sentence, and indicates that a noun phrase is Subject of the sentence, and also contrast or emphasis a noun pharse And this is what you asked... when "이/가" and "은/는" is used "subject 조사", it's hard to find the diffence. for example, "탐은 집 있다." "탐이 집에 있다" in english, both are same, "Tom is at home" let try considering the situation... If someone asked... "who is at home?" you can say "탐이 집에 있다" "where is Tom?" you can say "탐은 집에 있다. so if you want emphasis Tom's location, you can use "은/는" and when you use "은/는", there is another meaning depend on the situation.. in this case... "탐은 집에 있다", you can guess He is not makng trouble outside..he just stay at home.. It really depends on the situation.. let see anothere example "Seoul is capital of Korea" "서울이 한국의 수도이다." - A "서울은 한국의 수도이다." - B two guys are talking about seoul, one told "seoul is capital of japan" the other told "Seoul is captial of china" and you want to interupt.. which one is correct answer? A or B ? they are talking about "서울" you have to emphasis "서울" Yes.. the answer is B "서울은 한국의 수도이다" and two guys are talking about korean capital.. one told "tokyo is capital of korea" the other told "beijing is captial of korea" and you want to interupt.. which one is correct answer? they are talking about "Korean capital" so you don't have to emphasis "서울" so correct answer is A "서울이 한국의 수도이다."