A gentle one. She pushed the swing gently . She pushed the swing with a gently soothing rhythm. A subject complement tells something about the subject of the sentence. Because a subject is always a noun, only an adjective can be used as a subject complement.
Fear is irrational .
The roast beef smells funny . Quentin is not as happy as he appears. An object complement completes the meaning of the direct object of the clause. Because a direct object is always a noun or a pronoun, it can be modified only by an adjective. I declare the games finished .