이 and 가 can have multiple meanings, so we’ve been warned, and it’s necessary to think about these two in relation to 은 and 는. We started by learning it as a particle that marks the specific subject or introduces new information about the subject in question. Our teacher said one way of conceptualizing this is that 은 and 는 are “topic particles” whereas 이 and 가 are “subject particles” that really emphasize the subject more, introduce new information about the subject, or isolate it in some way.

This is admittedly wonky, especially after learning about 은 and 는 first and being told these concern what we would call the “subject” of the sentence in English. But consider a sentence like this, as a question: 카페가 어디에요? This question asks quite simply “where is the cafe?” I’ll admit it’s unclear if there is a specific reason that the subject marker of 가 is relevant here moreso than other reasons, but there are certainly multiple relevant reasons why 가 is appropriate. For one, it introduces the subject. We had not been talking about the cafe prior to the start of this conversation. Now we are. Second, it does—however implicitly—isolate the subject. Korean (I think?) doesn’t have the same definitive articles that we have in English, but this question is implicitly saying there is a singular cafe under consideration (“the cafe”). It has to be somewhere to the exclusion of any other place it could be. Thus, we’ve asked something specific and exclusionary about this particular cafe and the subject marker is relevant more than the topic marker.

Like 은 and 는, the choice of 이 or 가 is contingent on whether the noun in question ends in a consonant or vowel. 카페 ends in a vowel, so it gets 가. Were I to ask where the movie theater is (영화관이 어디에요?), the final consonant of the word for movie theater (영화관) compels the use of 이.

More to come, I’m sure.