은/는 vs 이/가 finally clicked for me

Sharing in case it helps someone. What worked for me: 이/가 points at WHO or WHAT (new or important info), while 은/는 sets the topic and often compares. So for '누가 왔어요?' the answer is '민수가 왔어요' (answering who). But '민수는 왔어요' sounds like Minsu came... and someone else didn't. Once I started thinking about which question the sentence is answering, it got so much easier. Still make mistakes all the time though.