The Importance of Inversions in English

If I were you, where are you? That's one of those inversions, not a question. If it was him who came into the class is technically correct. But funnily enough, this is one of those examples where I would probably say, if it was himWho Came In The Classroom Instead Of It Was He Who Messed Up Everything. Strictly speaking, proper grammar requires subject pronouns be used when they rename the subject. This is an example of me using the incorrect grammar because it's what I'm used to hearing.