은/는, 을/를, 이/가 을/를 is the object marker. They mark the object of a verb as in 나는 너를 사랑한다 (I love you). 은/는 is the topic marker and 이/가 the subject marker. Both can mark the subject of a sentence. Their difference is not so easily explained and takes time to fully grasp, but here's a somewhat simplified introduction. 1. 은/는 is the primary narrative tool to talk about something as the main topic.. 2. 이/가 plays a secondary role of bringing needed information to the context. For example, 1. 트럼프는 미국 대통령이다 (Trump is the US president) - talks about Trump as the main topic. 2. 오바마가 미국 대통령인가? 아니, 트럼프가 미국 대통령이다 (Is Obama the US president? No, Trump is) - supplies(answers) the information requested. #1 wants to say something about Trump, while #2 is supplying a fact to answer a question - this is the main distinction. Another example: 1. 나는 서울에 산다 (I live in Seoul) - straightforward sentence about "I" as the topic. 2. 내가 사는 곳은 서울이다 (The place where I live is Seoul) - 가 in sub-clause, 은 in main. #2 is a statement about "Where I live". "I" is only part of a sub-clause, not the main topic, so it's tagged with 가. Deriving from this difference of main topic vs incidental information, there are these secondary characteristics. 1. 은/는 has the effect of setting the subject apart from the rest of the group. 2. 이/가 can identify a particular item or member of a group. So, we see sentences like 나는 서울에 살고 친구는 인천에 산다 (I live in Seoul, where as my friend lives in Incheon). For 이/가, we may see: 어떤 남자가 있었다. 이 남자는 돈이 많았다 (There was a man. The man was rich). 가 in the first sentences identifies a particular man out of the indeterminate group of men to set as the topic. After that 는 is used to talk more about the established topic.

감사합니다

Talk to me in Korean explains some of it. http://talktomeinkorean.com/lessons/topic-subject-markers/ As for 를 that is an object marker. 예를 들어서: 영화를 봤어요. I watched the movie.