What is a predicate adjective?

A predicate adjective describes the subject of the sentence and follows a linking (intransitive) verb.

Examples

1. Alice Hamilton was intelligent.

The word "intelligent" describes Alice Hamilton and follows the linking verb "was," so it is a predicate adjective.

2. She was tenacious.

Subject: She

Verb: was (a linking verb)

Predicate adjective: tenacious (describes the subject).

Predicate adjectives vs. predicate nominatives

A predicate nominative renames the sentence's subject; i.e.  it offers another noun that denotes the subject.

Predicate nominatives, not like predicate adjectives, can be replaced by subjective pronouns.