"My student is a boy."
Translation:Discipulus meus est puer.
Discipulus meus est puer.
"discipulus meus" is the subject and is therefore in the nominative.
"est" because it's 3rd person singular of a stative verb, therefore there is no action to be received and no direct object, no accusative.
"puer" is the subject complement and is therefore in the nominative.