The adjective "green" in the phrase "a green tree" modifies "tree", and thus limits its meaning in that it cannot be, say, a deciduous tree in winter. In the same way, the adverb "kindly" modifies the past tense of the verb "let" in "she kindly let me borrow her scissors". An adverb may also modify an adjective, such as in "abjectly poor".
A premodifier is a modifier placed before the head (the modified component). A postmodifier is a modifier placed after the head. Example: "land (pre-modifier) mines in wartime (post-modifier)".
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." Understanding adverbial clauses and how they function in discourse is often very useful in interpreting subtle layers of meaning.
Another way of defining a modifier is that it, the adjective or adverb, is dependent on the part of the sentence it modifies, namely the noun or verb. Nouns and verbs are obligatory elements in that a complete sentence requires, minimally, a subject and a verb. Adjectives and adverbs, on the other hand, are optional elements. We can say, for example, "Dogs growl" (noun + verb) or "Big dogs growl loudly" (adjective + noun + verb + adverb). Either is a grammatical sentence, because the adjective and adverb are not essential in forming a complete sentence, whereas the noun and verb are.
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.