As described in the article The Functions of Adjectives and Adjective Phrases in English, prototypical English adjectives and adjective phrases can perform four grammatical functions: adjective phrase head, noun phrase modifier, subject complement, and object complement.

The article English Adjectives and Adjective Phrases: The Three Main Functions of Adjectives in the English Language explains three grammatical functions of prototypical adjective phrases in English: noun phrase modifier, predicate adjective, and object complement.

As described in the article Adjectives in English that Function Only Predicatively, some English adjectives can function only predicatively as subject complements and object complements.

Adjective Clauses

As described in the article The Forms and Functions of Clauses in English: Verb Clauses, Noun Clauses, Adjective Clauses, and Adverb Clauses, adjective, or relative, clauses perform the grammatical function of noun phrase modifier in English grammar.