"He is a teacher."
Translation:Er ist Lehrer.
Lehrer, in this sentence, is what is called a predicate nominative. Since "ist" works as basically an equal sign, Er = Lehrer, both nouns are in the nominative even though Lehrer is not the subject. For Lehrer to be accusative you would need a different verb like, "Er schlägt den Lehrer." "He hits the teacher." Accusatives receive action. Hope that helps.