은/는 vs 이/가 — the particle that trips everyone
Topic vs subject, explained without linguistics jargon
이/가 points at *who did it*. 은/는 sets *what we are talking about*.
누가 왔어요? (Who came?) → 민수가 왔어요. The answer needs 가, because the question is about identifying the subject. Using 민수는 here sounds like you are changing the subject.
은/는 also carries contrast.
커피는 좋아해요 means *coffee, I like* — with an unspoken *…but something else, not so much*. Native speakers hear that contrast even when you did not intend it.
Rule of thumb that survives real conversation.
New information → 이/가. Already-known topic → 은/는. First mention of a person often takes 가; the second mention usually switches to 는.