From the mailbag:
I’ve had more questions about when to use “me” vs. “I.” What you really are asking is how to differentiate between the objective and subjective case of a pronoun. If the pronoun follows a verb or a prepostion, it takes the objective “me” form.
Give me the book. The book belongs to me.
If the pronoun is performing the action, it is the subject of the clause and requires the “I” form.
Am I the volunteer? It is I.
Back in the day, when we diagrammed sentences, we had a visual of the noun-verb-object (or modifier) that made it easy to choose the correct pronoun form.
I | is \ it
I miss diagramming and still diagram sentences in my head. 
Cheryl
Do you use “It is I”? It sounds quite stilted.
Mar, candidly I would NEVER say “it is I” because I don’t speak that way *LOL*. You are right. It sounds stilted. Correct grammar should be user-friendly. I would reword the sentence to something like “I am the one.”