You knock on the door of your parents house. You have come for the holidays and are eager to get the families together. Your mother comes to the door but does not open it. Instead, she calls, "Who is it?" You know that if you say, "It's me," she is going to have words with you over your illiterate English. You choke out what you believe is the correct way to say it, "It is I, Mom." She lets you in and compliments you on your understanding of English grammar. I am here to tell you that you still did not get it right. You may have satisfied your retired English teacher mother, but you still committed a bad grammar mistake.
The response, "It is I," sounds really stiff to most English speaking people, and yet, not many have taken the time to figure out why. It sounds all wrong for the same reason that "It am I," is wrong. If you take a good look at both of these answers, you will see what the problem is. It all comes down to matching the person of the pronoun with that of the being verb.
For instance, saying, "It am I," tries to match the third person "it" with the first person "am." Of course, this does not work out. When saying, "It is I," it tries to match the third person "is" with the first person "I," and this also does not work out. Unfortunately, most Grammarians have touted this form as correct. It just shows you how deeply the "experts" are willing to go in thinking out their analysis—not far enough!
There is a simple solution to this. "I" must be used with "am," and "it" must be used with "is." The sentence will have to introduce a little bit of extra stuff into it in order to get an answer that actually works. The two clauses, "I am" and "it is" can be connected with a simple pronoun as follows: "I am who it is." In this case, "who" is a function word to connect "I am" to the relative clause "it is."
Now, for the answer in the above scenario. The mother says, "Who is it?" Then, you answer, "I'm who it is, Mom." This sounds quite nice and doesn't give it that stiff, stuck-up quality. Your mother has gotten a good opportunity to hear and consider your voice, so she can determine who it is and also approve of your usage. Problem solved!