Subject complements are used only with a class of verbs called linking verbs or copulative verbs, of which to be is the most common. Unlike object complements, subject complements are not affected by the action of the verb, and they describe or explain the subject.
Examples of sentences with subject complements:
The lake was a tranquil pool.
"Was" is a linking verb which links the subject complement (predicate noun modified by an adjective) "tranquil pool" to the subject "lake."
The lake is tranquil.
"Tranquil" is a predicate adjective linked through the verb "is.
However, many prescriptive grammarians, like Norman Lewis, frown upon this usage and regard it as a mistake. 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 is 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."
Among older fiction writers, characters sometimes speak in an ungrammatical way, but an authorial note will then point this out. In "The Curse of the Golden Cross," G. K. Chesterton writes,
"He may be me," said Father Brown, with cheerful contempt for grammar.
"Come out, Mrs. Beaver. Come out, Sons and Daughters of Adam. It's all right! It isn't Her!" This was bad grammar of course, but that is how beavers talk when they are excited.
One should say "who is it?", as opposed to the incorrect "whom is it?". This often causes confusion when explaining, as the more infrequent usage of "It is I", opting instead for "It is me" would imply that "me" is the object of the verb "to be", and therefore "whom" ought to be employed.
Perhaps the simplest example is the existential question "Who am I?" It shows both questions. It should be apparent to most that "Whom am I," "Who is me," and even worse, "Whom is me" are all incorrect.
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 is me) not c'est je. Here, moi is a disjunctive pronoun, or less technically, a ''stressed pronoun.