A predicate adjective is one of many different types of adjectives. Basically, predicate adjectives modify the subject of the sentence. Simple enough, right? The only other thing to note is that these adjectives are always connected to the subject by a linking verb.
Here's an example. In the sentence "The wall is purple," the subject is "wall," the predicate adjective is "purple" and the linking verb is "is." So, it's subject, verb, and predicate adjective.
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