In fact, the last time a phone made me feel dumb was when I realized I had no idea why people answer “This is he” instead of “This is him” or “This is she” instead of “This is her.”
I knew that people often opt for the subject pronoun (he or she) when they wanted to sound more formal, and that they may stick with the object form (him or her) when they are being more casual. But I couldn’t figure out the grammar of it.
It seemed to me that the object form was correct. After all, that’s the form used in so many similar sentences. I called her. I watched him. I knew them. The old subject-plus-verb structure just begs to be followed by an object. So “This is she” made no sense to me.
It took some digging, but eventually I found out that the answer has to do with something called the predicate nominative. And don’t let the name put you off. It’s actually a pretty simple concept.
Any time you have a noun or pronoun followed by a form of “be,” the thing that follows is called the predicate nominative. The predicate nominative literally renames the predicate (the first noun or pronoun). Look at the sentence “The butler was the murderer.” Here, the predicate is “the butler,” the verb is a past-tense form of “be,” and the noun that follows refers back to butler. In this case, “the murderer” is the predicate nominative. It’s the subject renamed.
Of course, a sentence like this isn’t confusing because “the murderer” doesn’t change form according to whether it’s a subject or an object. The murderer watched TV. The cops caught the murderer. It’s the same in either case.