Use 은/는 to set a topic or contrast
은/는 often means ‘as for …’ in the background, even when English does not say those words. It is also common when contrasting one thing with another.
As for me…
Today, in contrast with another time.
Use 이/가 to identify or introduce the subject
이/가 is especially useful when answering ‘who?’ or ‘what?’ or when presenting new information. It points to the noun that is doing or being something.
Why both can be correct
Compare ‘제가 할게요’ with ‘저는 할게요.’ The first naturally identifies ‘me’ as the person who will do it. The second sets ‘me’ as the topic and can carry a contrast such as ‘as for me, I’ll do it.’ The grammar is not just a mechanical choice; information focus matters.
Questions often reveal the natural choice
Imagine the unspoken question before the answer. If the question is ‘Who came?’, the answer naturally identifies the person with 이/가. If the conversation is already about a person and asks what they are like, 은/는 can keep that person as the topic.
제가 identifies the answer to who.
Minsu remains the established topic.
Contrast can be stated or only implied
은/는 can compare two items directly, but it can also imply a contrast without spelling out the other side. ‘커피는 좋아해요’ may suggest that coffee is liked while something else is not, depending on the conversation. Do not add contrast to the English translation unless the context really supports it.
The contrast is explicit.
은 can gently contrast today with another day.