은/는 is placed after a subject, 이/가 is placed after a (subjective/objective) complement, add to that 을/를 is placed after an object in detail : 은/는 은 - when the subject ends with the final consonant 하늘은 파래요 (sky colors blue) 는 - when the subject ends without the final consonant 나는 멋져요 (i am cool) 이/가 이 - when a complement ends with the final consonant 수탉이 날아요 (a cock is flying) 나는 공산당이 싫어요 (i hate communists) 가 - when a complement ends without the final consonant 나라가 어수선해요 (country is being messed up) 쟤는 내가 찜했어요 (I poked her/him) 을/를 을 - when an object ends with the final consonant 곰이 마늘을 먹어요 (a bear is eating garlics) 미국은 이스라엘이 팔레스타인을 공격하는 것을 못 본 체 해요( US pretends not seeing that Israel attacks Palestine) 를 - when an object ends without the final consonant 사자가 하이에나를 공격해요 (a lion is attacking a haiena) 아빠는 내가 공부를 해야한대요 (my dad says that I have to 'do' study) there is 에게/게 for an indirect object as well hope this helped

I'm not Korean, I'm native English, but I learned how to use 은\는 by thinking of them as "is, are, am, etc" (They are used other ways but this is how I first learned to use them.) Example: I am bored = 나는 지루해. I am going to school = 나는 학교에 간다. It's not always used like that though >o<; Example: I like cats = 나는 고양이들을 좋아해. I love you = 나는 너를 사랑해. I don't know about 이\가 but they can be used after names instead of 은\는 Aàdèlio Astèllà can explain the rest. ^^; Example: (These are Korean names) 지현이 = Ji-Hyun-Ee 잔디가 = Jan-Di-Ga