는/은 are topic/subject particles. 는/은 is used for a subject or topic of a sentence. 는 is used for nouns without 'patchim' (final consonant), and 은 for nouns with a final consonant. For example: +나는 = I am (나 is a word without a final consonant, therefore 는 is attached to it). 나는 학생이다 = I am a student +하늘은 = The sky is (하늘 is a word with a final consonant ㄹ,therefore, 은 is attached to it.) 하늘은 높다 = The sky is high

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