1.subject marker (이/가) is meant to focus a spotlight on the noun. If there is no previous context, a good default is to use the subject marker.
Example 1: If I suddenly take out a book and a pen, and hold the pen above the book and ask you
to describe the overall situation, the best response is:
펜이 책 위에 있어요
2. The topic marker 은/는 has two uses. (It has more, but these two are the basics.)
a. It is used for contrast. You were speaking about this, but now you are speaking about that.
Continuing from Example 1, if I am still holding the book and then I reveal that there is a pencil (연필) behind it, you might say:
연필은 책 뒤에 있어요.
The pencil [in contrast to the pen] is behind the book.
b. It is used in situations when the topic is probably obvious to the listener, where you could have dropped the noun altogether, but just to be clear you want to gently remind the listener of the topic.
The first sentence below introduces the pen into the spotlight with the subject marker. In the next sentence “pen” doesn’t need repeating, since it is clear from the context. By the third sentence, it is used as a gentle reminder with the topic marker, so the listener knows we are still talking about the pen, not the pencil. Using 펜이 there would sound odd, as if it weren’t the same old pen you were previously talking about.
펜이 책 위에 있어요. 연필 옆에 있어요. 펜은 우리 아버지 거예요.
The pen is above the book. It is next to the pencil. The pen is my father’s.
This is why you tend to hear “나는” (“I”) more often than “내가”. As you continue in a series of sentences to talk about yourself, you continue to be the ongoing topic of your sentences, so you use the topic marker 나는 — if you need to mention yourself explicitly at all. You also might want to contrast yourself with someone else, as in 2a. That calls for 나는 as well.
In fact, in real world language use, discourse flows and topics persist and shift with the flow. This is why topic markers are so common. In fact, the primary sentence structure in Korean is “topic + comment.”
