이/가 - emphasizes subject 은/는 - emphasizes action/following sentence _____________________________________ This thing is a desk... 이것이 책상이에요. --> Emphasizes 'this thing' 이것은 책상이에요. --> Emphasizes 'is a desk' Therefore, it might also be explained as... 이것이 책상이에요. --> THIS THING [as opposed to a another, different thing] is a desk. 이것은 책상이에요. --> This thing iS A DESK [instead of something else, like a book] _____________________________________ Where is the restroom? 화장실이 어디에 있어요? --> Where is THE RESTROOM (as opposed to someplace else, like the kitchen) 화장실은 백화점에 있어요. --> The bathroom is AT THE DEPARTMENT STORE. (as opposed to someplace else, like school) Therefore, it might also be explained as... 화장실이 어디에 있어요? --> Use 이 to emphasize that it is THE RESTROOM that we are looking for. 화장실은 백화점에 있어요. --> DON'T use 이 because we already know we're looking for the bathroom. Instead, use 은 to emphasize the location of the restroom we are looking for, AT THE DEPARTMENT STORE. ____________________________________ Need more help? Go here for additional information: http://www.youtube.com/watch?v=xSpoLuJ42zA&feature=relmfu

they are used as subject particles