Language is a fluid, living social construct. The rules of grammar were not carved on stone tablets and handed down by God. They were created by human beings who had observations about how language works, and opinions about how it should work.
“Subject pronoun,” “predicate nominative,” and the like are almost insider terms, ones that many people forget shortly after learning them in school. As we say, knowing why those rules exist, and deciding whether to apply them in that situation, is more important than just following the rules blindly. —Columbia Journalism Review.