Sorry for the late post!
I used to watch foreign films, some of which had English subtitles at the bottom of the screen. It was nearly 10 years ago, but I still remember this puzzling moment: The scene took place in the days of French Revolution. A warrior called out loudly to open the gate into the city. The gate keeper replied by asking for the warrior to identify himself. The warrior replied, "It is I, so-and-so." It is I...? Why? I thought it was, "It's me." I don't remember the title of the movie or anything else before or after that scene. Thinking that my French teacher must know grammar since she knowa French, I asked her why. SHe didn't know. My English teacher didn't know either!! Internet was just starting up then so it wasn't a readily available source yet. I started college the following year, and it was then that I finally solved the problem. Pronoun takes an object case after a verb but not when the verb is a to be verb. So it makes sense when we say, "This is she" on the phone. Mystery was solved!