Korean grammar is one of the reasons that Korean is so hard. Completely alien to native English speakers, particles, complex verb endings and the like continue to plague Korean learners right up to advanced levels.
That said, learning the foundations well can make a huge difference so make sure you follow the grammar cards.
은 (eun) / 는 (neun)
The main point of a topic marking particle is to indicate the topic, to let people know what you are going to talk about.
Nouns ending in a consonant : 은 (가방+은)
Nouns ending in a vowel : 는 (사과+는)
*The topic of the sentence is usually (but not always) the same as the subject of the sentence.
저는 학생이에요. (As for me) I am a student.