The Topic Particle 는 and 은
The topic particle can be added to a noun to or another particle to indicate that the marked word is the topic of the clause. When added to a particle or noun that has a , 은 is added to the word otherwise 는 is added to the word. This may be used to re-introduce a new topic or a topic that was previously mentioned in conversation.
The topic particle can also used to express contrasts.