It is very confusing and difficult for foreigner to understand it. Even many korean also cannot explain it exactly. The noun is always combine with postpositional particle in korean sentence . Grammatically, 이/가 is a subjective particle to make a noun subject. and 은/는 is a assistant particle to help meaning something. But 은/는 is used as if subjetive particle more often in a sentence. They are almost same in meaning, nuance except some case. I think it will be better to study differences in advanced level.

You already have a good answer for 은/는/이/가 from Michael. I want to add more detail to '-은' for reference. 받침 refers to the consonant placed in syllable of word. 1.은 is linked to a noun or pronoun that has 받침, or is linked to 부사어(a word functioning as an adverb in sentences) or compound verb in Korean for representing one is in contrast with another. 인생은 짧고 예술은 길다.(Life is short, art is long.) 이 방이 깨끗은 하지만, 너무 좁다.(This room is clean, but too small.) 2.은 is linked to a noun or pronoun that has 받침, and used for representing a thing is the topic.) 오늘은 날씨가 맑다.(It's sunny today) 이 책은 내 여동생이 도서관에서 빌려 왔다.(My sister borrowed this book from library.) 내가 그 친구를 마지막으로 본 것은 작년 여름이었다.(It was last summer I saw the the friend for the last time.) 3.은 is also used for emphasizing. 너에게도 잘못은 있다.(You, too, have a fault.) 무엇이든 열심히 노력하면은 좋은 결과가 있을 것이다 (It will brought about a good result if you try hard anything.)

Thank you for not helping at all~ eh, you're right but I've already read so much but I still confuse it -.- stupid.

I would suggest you do a google search. There's so much information about this already. It's probably the most asked question about Korean.