The traditional rule is that personal pronouns after the verb to be take the nominative case (I; she; he; we; they). Some 400 years ago, me and other objective pronouns (him; her; us; them) began to replace the subjective forms after be. Today, such constructions - It's me. That's him. It must be them -are almost universal in informal speech. In formal speech and in edited writing, however, the subjective forms are used: It must be they. The figure at the window had been she, not her husband. The objective forms have also replaced the subjective forms in speech in such constructions as Me neither. Who, them?