Objects of “to be” verbs must be in the subject form. Watch for pronouns following “to be” verbs (these include verbs such as “it should have been,” “it is,” “it could have been,” “it was,” etc.), and make sure they are in subject form. Note that this is an exception to the rule: normally, you would use the object form when the pronoun is the object of the verb.
This is another error common in speech, but it is simple enough to identify and correct as long as you are able to:
1) Identify a “to be” verb.
The verb “to be” takes many forms, some of which can be difficult to recognize as “to be” verbs. Look for the words am, are, is, was, and were, as well as constructions involving the words be, being, and been.
2) Remember the subject form of the pronoun.
Refer to the chart below for the different forms of personal pronouns.