The 은/는 particle can also be used to refer to something mentioned earlier in the conversation or to evoke something already known by both sides in the conversation.
The 은/는 particle is used to express comparison or contrast. In this case, it can attached to more than just subjects.
The particle 은/는 is also used to emphasize on something.
When using formal language, we use the particle
When spoken, the
Mark the subject, who or what is doing the action.
Mark the topic, what the sentence is about. The topic of the conversation.
Mark the object, who or what is being acted upon.
Note that these particles can sometimes be dropped in practice and we assume the audience is smart enough to understand the context.