Although commonly called “the subject particle”, 이/가 isn’t really a subject marker. In order to be correctly called a subject particle, subject marking should be the only thing that it does, and that isn’t the case. Particle 이/가 is used for non-subject marking purposes and sometimes marking subjects with 이/가 can be inappropriate. And so we need to question what 이/가 essentially is and why speakers use it.

I’m not trying to be provocative when I say “이/가 is not a subject particle”. This is a known fact. Nobody (advanced learners, linguists and grammarians) seriously considers 이/가 to be a subject marker. It’s a term that Korean teachers use for simplicity. However this simplified term doesn’t help Korean learners because in order to use this particle properly, we need to see there is more to this particle than “subject marking”.