New Testament Greek for Beginners (Lesson VIII - audio samples)

99. The verb εἰμί takes a predicate nominative, not an accusative, to complete its meaning.

Examples (audio):

ὁ ἀπόστολος ἄνθρωπός ἐστιν, the apostle is a man;
ὁ ἀπόστολος ἀγαθός ἐστιν, the apostle is good.