That doesn't explain those postpositions clearly even correctly. Just forget it. -이/가 put emphasize on the next word(a verb, a noun, an adjective), and -은/는 put emphasize on the subject. 한국말이 *재미있어요*. Korean is INTERESTING. *한국말*은 재미있어요. KOREAN is interesting. These above aren't very natural to say by theirselves as the two words are emphasized with no giving the context. 이것이 *연필*이에요. This is A PENCIL. *이것*은 연필이에요. THIS is a pencil. For your information, we usually say 이거(이것), 이게(이것이), 이건(이것은). Korean speakers don't always use postpositions. Just know some situations you should use it. When you introduce yourself to people, "저는 Avann이예요." is most natural. By now, you'd know '는' emphasizes '저' in it. To put emphasize on something earns a profit in general. Getting attention from other people. And if you're going to say something opposite to what someone has said right before you speak, using one of postpositions would make your Korean shine. A : Who broke this? 이거 누가 깨뜨렸어? (to B)You did it, right? 네가 그랬지? B : I didn't do it. 내가 안 했어. If B answered "난(나는) 안 했어.", this implies someone else did it. Hope this helps.

i have the same doubt, so i did some looking and this is what i got from a grammar book. '-이/가' is used as subject marker,hence indicating the syntactic role (e.g. whether the noun is a subject, an object, indirect object etc.) of the noun preceding it as the subject eg. 이것이 연필이에요. This (subject) is a pencil whereas '-은/는' is topic marker, which function is not to indicate syntactic roles of the noun but rather marking the noun as the topic of conversation/statement, emphasizing the singularity of the noun. eg. 이것은 연필이에요. As for this, its a pencil (emphasis on 이것) they can appear in the same sentence too. eg. 햄버거는 스미스 햄버거가 맛있어요 (as for hamburgers, Smith Hamburger is tasty) notice that hamburger is the topic of the sentence whereas "Smith Hamburger" is the subject of the predicate "tasty"

I was researching about the topic and it is not a simple matter. In short, it is used like this. 1. A bit like a vs. the. 2. 은/는 is used to contrast. 3. The focus of 이/가 is on the subject. The focus of 은/는 is on the topic/description. 4. 은/는 can be used with other markers or at the end of a verb conjugation 5. When the sentences are combined, 이/가 must be used in the sentence in the middle. For more information, you can visit this link, http://wiseinit.com/topic-vs-subject-marker