이/가 and 은/는 is a big subject as they are used everywhere all the time, and often with subtly different connotations. It is not something you can explain on a page or two. Here's some starter notes. 이/가 is simple. It marks the subject ("subject marker") of a sentence like in other languages. * 차가 온다: A car is coming. * 사람이 쓰러져 있다: A man is left lying there. 은/는 is a "topic marker", but sometimes functions as a "subject marker" as well. When it introduces a subject like 이/가, it adds the connotation of starting a new topic, drawing the listener's attention to it. It can also start a topic so that a full subject+predicate can follow. * 나는 학교에 갔다 I went to school - a simple case similar to 이/가. * 나는 키가 크다 "As for me, my stature is high" - has both topic & subject markers . In both cases, 는 opens a topic, focusing the listener's attention on 나, to say something about 나. In "나는 키가 크다", there is a topic(나), subject(키), and predicate(크다). It is a special kind of compound sentence unique to Korean and Japanese. To say the same thing in English, you'd say something like "As for me, ...". 은/는 brings about many different nuanced usages. It makes up one of the most important characteristics of the Korean language. * 나는 괜찮은데.. I am Ok.. (it suggests "what about you?") * 그가 왔을 때 나는 자고 있었다 (typical sub-main clause case: 이/가 in sub-clause, 은/는 in main) 은/는 can also attach to an adverb to direct the attention to that part, and thus invites the listener to think about the other cases - it sets it up for a contrast effect. * 거기 가지 마. Don't go there - simple and no extra connotation. * 거기는 가지 마. (는 on an adverb) Don't go there (but perhaps go somewhere else) This subject has been asked and answered at the link below. The linked page has a link to a grammar site you can check. http://www.italki.com/question/316973