I'm studying Japanese and recently I came across the term "double nominatives". The idea is that ga marks the nominative case, so a phrase with two ga has two phrases in the nominative case:
boku ga eiga ga suki da I NOM movies NOM like COP I like movies
I understand that the copula da is supposed to connect the subject to the complement. Since there's only one copula, it makes sense to think that there's only one subject. However, two phrases are marked as nominative. So one of them is nominative AND subject, and the other is ONLY nominative and NOT the subject.
My question is: if the nominative case isn't necessarily the subject, then what is the nominative case, if not the subject?
Note: I also understand that in the phrase above, we can attribute other functions to the first phrase, like experiencer, or focus. I think I've seen some studies claim ga marks either the subject or the focus. But now I'm seeing studies saying it marks the nominative case in both instances. I'm trying to understand how double-nominatives are supposed to work.