Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
it’s me
Most speakers of English tend to put nominative case pronouns at the left-hand side of the clause, in “subjective” territory before the verb, and objective case pronouns at the right-hand side of the clause, in “objective” territory after it. Apparently the pressure of this habit is so great that it overwhelms the Standard Formal pattern for the special class of verbs called linking or copulative verbs, wherein It is she is required, at least by rule, rather than It’s her, or where This is he is needed, not This is him. The primary use of the objective case pronoun after linking verbs is in the first person: It’s us, It’s me. With third person, singular and plural, many Standard speakers will retain the nominative, even at lower levels of speech and in Informal uses. (And of course with second person you, the nominative and objective are indistinguishable.) But It’s me and It’s us are both Standard in all Conversational and most Informal uses, perhaps in part because they occur almost exclusively in speech anyway. Consider the way you answer the phone if the caller asks for you. To a stranger you’ll respond (if you’re a Standard speaker), This is she [he], not This is me, or you’ll dodge the issue entirely and say Speaking. If you know the caller well, though, It’s me will serve. In Oratorical speech and Formal writing, however, Standard English demands the nominative: It is we who must shoulder the burden. It is us just won’t do in that sort of context.
