I would like to add some stuffs about 은/는. Actually when we say subject particles, we mean 이/가, 에서, 께서 and 서. 은/는 is what is called a comparison auxiliary particle, but can also be used like a subject particle. It is really hard to tell when to use which eventhough I am a Korean. In verbal sentences 이/가 is mostly used to emphasize the subject. It is kind of like 'Not anythung else but the SUBJECT', but like I said, not always. 은/는 is more like a state, like I am a state of "doing something". It is a comparison auxiliary particle because when we say '나는 사과를 먹는다 (I eat an apple)', it is saying '(Maybe others are doing something else, but) I am eating an apple'. Why do we say like this? I DON'T KNOW, but I suppose that it came from our cultural thoughts and unconsciousness. I say, if you want to use the right particles, you better read a lot. Until then, you may just think that 은/는 means to be or 'be' verb + ~ing