The word modifier applies to either the adjective or the adverb in a sentence. An adjective is not actually required but helps modify and limit the extent of the meaning of a noun or pronoun, while an adverb helps modify a verb in a similar way. Thus modifiers, though not actually necessary help define the meaning and are attributes of nouns, pronouns or verbs. A modifier is also called a qualifier.
The adjective "green" in "a green tree" modifies and thus limits the meaning of the noun "a tree" in that it cannot be "a deciduous tree in winter", as the adverb "kindly" modifies the pasttense of the verb "let" in "she kindly let me borrow her scissors". An adverb may also modify an adjective, such as in "abjectly poor".
Adverbial clauses (or particle phrases) such as "of course", "as it were", etc., commenting on the rest of the sentence or what has gone before in a previous sentence, may also be classed as modifiers, as in "Of course, he was never one to be silent" or "Unfortunately, we arrived late".
Another way of defining a modifier is that it, the adjective or adverb, is more dependent on the part of the sentence it modifies, namely the noun or verb. The noun is independent and may exist on its own, such as "the tree" which then may be modified by "green", or the verb being independent as in "work" in "I work (every day)" may be modified by the adverb "hard" in "I work hard (every day)".
In compound nouns, the first of the two words so combined functions as a modifier, such as "elementary" in "elementary school", "mountain" in "mountain bike", etc.
Lastly, modifiers may be divided into pre-modifier + subject(noun) + post-modifier, as in "land (pre-modifier) mines in wartime (post-modifier)".
