A modifier describes another word or word group. Words, phrases (groups of words without a subject or predicate, usually introduced by a preposition or conjunction), and clauses (groups of words with a subject and verb within a compound or complex sentence) may all be modifiers. An adjective modifies a noun or a pronoun. An adverb modifies a verb, an adjective, another adverb, or a clause. Clauses or phrases may serve as adjectives or adverbs.