Very simple: objects of to be verbs are in the subject form. Watch for pronouns following "to be" verbs such "it should have been," "it is," "it could have been," "it was", and make sure they are in subject form. This is another error common in speech; but it's easy to identify.

Incorrect: It must have been her who called.

Correct: It must have been she who called.

"It must have been" is a "to be" verb, so the pronoun must be in subject form: "she," not "her".

E. Possessive Pronouns

G. Relative Pronouns