The principal use of the nominative case is to identify the subject. The subject
will usually be a noun, a pronoun, or an adjective or participle used as a
noun, though in Greek often the subject is unexpressed, being included in
the form of the verb. If the subject is expressed (in any of the four forms
above), the word must be in the nominative case.
In a sentence with the predicate "to be," other nominative case
nouns (or noun-like things) may be included. If so, these must refer to the
same item as the subject does. In the sentence: "The man is a
priest", the words 'man' and 'priest' would both be in the nominative
case, and clearly refer to the same person.
Think of the predicate
"to be" as an EQUAL SIGN. It has no direct object. What is before
or after the predicate usually refers to the same person or thing, and in
such cases both are put in the nominative case.