Modifiers are words or groups of words that limit, enumerate, or describe and clarify the meaning of other words—called headwords. The headwords are nouns (or words acting as nouns) or verbs. Modifiers act either adjectivally or adverbially.
Here are some examples of modifiers, with their names and functions shown:
The large house suddenly darkened. (one word adjective large modifying the noun house) (one-word adverb suddenly modifying the verb darkened)
The house on the hill frightened the children. (prepositional phrase acting as an adjective to modify the noun house)
The case, which was decided by the Supreme Court, desegregated the public schools. (nonrestrictive adjectival clause modifying the noun case)