“It is I” vs. “It is me”

The reason the verb "to be" is an exception is that its meaning makes it equivalent to an equal sign. "It is I." means: It = I.

Both "It" and "I" are co-equal subjects of the sentence. There is no object. The subject of a sentence, in this case both subjects, require the nominative case.

Contrast this with the sentence : "It hit me." The subject "it" acts upon the object "me," so the objective case is required.

Another example of the exception with the verb "to be", which may be surprising, is: "It was we." This is the correct usage for the same reason, however in common usage, most people say, "It was us," which is technically incorrect.

"It is I" is correct, grammatically correct, that is; however, I would never say that unless I were being waterboarded, and I think that most people feel the same way. I won't get into predicate nominatives, linking verbs, and all that stuff, but "It is I," though sounding stilted and pretentious to most of us, is correct, dammit!

The simplest way to explain it is by first identifying that the first word “It” is the subject. In the sentence, the person speaking / writing is simply restating the subject by using the appropriate specific subjective case pronoun. So, “ It is I,” is correct.

I think the right way of understanding this problem is somewhat similar to the French usage of "tonic pronouns" after "c'est" (it is) or after prepositions. Some of the French tonic pronoun forms are different from the normal subject form (moi/je) but not all (elle/elle). When we say "it's me" or "it's her" in English what we're doing is using the "tonic" form. At least that's how I explain to myself. No need to think that the impossibly pedantic "it is I" is the correct form.