은/는 is a topic marker, you can take it as 'as for...' / 'talking of...' / 'as opposed to others' / 'in one's case'. A : 난 그 노래 별로야. I don't like the song that much. B : 난 좋아. In my case, it's good. (Simply, "But I like it") A : 술 한 잔 하러 가자. Let's go for drinks. B : 나 돈 없어. I have no money. A₁: 나 돈 있어. I have money. A₂: 난 돈 있어. I have money (unlike you / even though you have no money) A : 너 나 사랑해? Do you love me? B₁: 좋아는 해. I LIKE you. (but I might not love you) B₂: 넌 사랑해. I love YOU. (but I don't love her/him or someone else)

Here is thing, '은/는' is called 'subject marker', placed after 'subject' , and if the last word of subjects has 'a pat'chim', usually '은' follows. For example, 나는 한국인입니다. '나'(subject) as in '나는' doesn't have a pat'chim,so that '는' followed. 하늘은 파랗다. as in this sentence, '늘' has a pat'chim(ㄹ). '이' and '가' also follow the same rules. I think techincal explanations will make you more confused, and it's pointless as well. also it's not easy to memorize all rules except for the above exaplanation. The best way is to get used to it lots of sentences, I guess.

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."

thank you very much. your explaination is simple,but i get.

名词 + 이/가 1. 以元音结尾的名词后 + 가 2. 以辅音结尾的名词后, + 이