은/는 are topic particles, and 이/가 are subject particles. Let's start with 이/가. In English, a subject is a noun that conducts the action of the verb. This is almost always the case in Korean, too: the subject is whatever carries out the verb. 예:민지가 파스타를 먹어요. Here, the subject "Minji" is carrying out the action (eating); exactly as it is in English. 예:장미가 너무 예쁘죠? In this sentence, the Korean subject isnt the same as the English one. The word marked with the 가 (rose) doesn't carry out any action at all. Instead, it's the referent of the verb. In Korean, a subject doesn't always carry out an action: sometimes its just the word that the adjective at the end of the sentence describes. Now, lets talk about topics. Remember how in middle and high school English classes you would write essays &every paragraph was supposed to begin with a topic sentence? The topic sentence was usually at the beginning and it summarized the central theme of each paragraph. If you removed every other sentence from the paragraph, you'd lose the details but you'd know what the paragraph was mainly about. This is how the topic markers 은/는 work. They tell you what the sentence is about. 예: 저 사람은 내 룸메이트예요. If I took all the details but the topic out, you'd still know that we were talking about "that person." You may not know what I wanted to say about them, but you'd know what the topic of conversation was.

Google it. It has been answered a million times before.

Thank you Classiko this Blog really helps me. Thanks so much. (:

here is a very good Blog : http://wildkorean.blogspot.com/2011/12/topic-marker-and-subject-marker.html ------- http://wildkorean.blogspot.com/2011/12/transitive-and-intransitive-verbs.html