It's I/It's me
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It is commonly held that in the statement "It's I" (or "'Tis I"), "be" acts as a transitive verb and thus, I would be incorrect since it should be the object, and the objective case me should be used. In fact, in terms of common usage, especially in informal speech, "It's me" is rather common.
However, many prescriptive grammaticans frown upon this usage and regard it as a mistake. Technically, the I in "It's I." is a subject complement. Subject complements are used only with a class of verbs called linking verbs or copulative verbs, of which to be is the most common. (others being to become, to remain, etc.) Unlike object complements, subject complements are not affected by the action of the verb, and they describe or explain the subject. In this case, I is not affected by the action of the verb is, and it specifies exactly who the subject It is. In formal English, the subject complement therefore takes the subjective case. Usually, this makes no difference in the sentence because English nouns no longer distinguish between subjective and objective case. But English pronouns make the distinction, and the subject complement takes I instead of me. "It's I" sounds strange to many English speakers, but is considered correct by prescriptivists. In other contexts, the subject complement may sound less strange, such as "This is she" rather than "This is her."
For this reason, one should say "who is it?", as opposed the incorrect "whom is it?". This often causes confusion when explaining, as the more infrequent usage of "It's I", opting instead for "It's me" would imply that "me" is the object of the verb "to be", and therefore "whom" ought to be employed.
At this point, the use of the subjective in the subject complement has almost entirely disappeared. Both usages are still current, but the use of subjective in the subject complement is much less common.
The use of a nominative complement ("It is I") is by no means universal in other languages. For example, French-speakers say "c'est moi" (it's me) not "c'est je". Here, "moi" is a disjunctive pronoun, or less technically, stressed pronoun.
