은/는 marks the main topic of the conversation OR makes some sort of contrast with something else 나는 돈이 없어요. I (main topic) don't have money. OR 동생이 돈이 없어요. 나는 돈이 있어요. my younger brother has no money. I (in contrast to my younger brother) have money. 이/가 marks the subject of the verb. 선생님이 가요. Teacher is going. 을/를 represents the object of the verb. 음식을 먹어요. I eat food. 이/가 and 은/는 can sometimes be used interchangeably, so don't worry too much about them. for your examples: 저는 학생이에요. 그래서 돈이 없어요. depending on the context, you could be talking about yourself as the main topic of the conversation OR contrasting to someone else who is not a student. 저는 학생이에요. 그래서 돈은 없어요. in this case, my feeling is that 저는 shows you are the main topic, but 돈은 shows that in contrast to a car or shoes or whatever that you do have, you do not have money.