A linking verb can have two different kinds of complement:
- When the complement is a noun, it is called a predicate nominative.
- When the complement is an adjective, it is called a predicate adjective.
A noun functioning as a predicate nominative is always in the nominative case.
You are nice.
“nice” is a predicate adjective; since it is an adjective, not a noun.
You are a hero.
“hero” is a predicate nominative; since “hero” is a noun, not an adjective. See lesson 60 here.