A second use for the nominative case

2) The nominative case is used for a predicate noun following a linking verb.

This is easier than it sounds in grammar-speak. In the sentence

Ο απόστολος βλέπει τον προφήτην.
The apostle sees the prophet.

only 'απόστολος' is in the nominative case, as subject of the verb. 'The prophet' ('τον προφήτην') is in the accusative case, as the direct object (what is being seen) of the verb.

But in the sentence

Ο απόστολός εστιν προφήτης.
The apostle is a prophet.

both 'απόστολος' and 'προφήτης' are in the nominative, because the verb 'εστιν' is a linking verb: it doesn't describe what the subject does, it describes what the subject is. The apostle is a prophet. In this sentence 'προφήτης' is a predicate noun, or predicate nominative.

The verbs that can serve as linking verbs include:

ειμι ('I am')

and

γίνομαι ('I become').

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