"A seer is a holy man."
Translation:Vates est vir sacer.
Sentences with a form of esse (to be) as the verb, and nouns on both sides (like "A est B" or "C sunt D"), take nominative nouns on both sides. Any adjective associated with either noun (e.g. "sacer") is thus also nominative (and as usual, also agreeing in number and gender).