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_.