TO BE, UNITING WORDS.
Words united by to be, referring to the same person, must be of the same case.
"It is me," "It may have been him," "It could not be her," and "It was not them," are not correct: it, in each of the sentences, is nominative and the other pronouns should be I, he, she and they. "I took it to be he," and "I understood it to be they," are also wrong; for it is objective in both instances, and the following pronouns should be him and them.