27b Use adverbs to modify verbs, adjectives, and other adverbs.
When adverbs modify verbs (or verbals), they nearly always answer the question When? Where? How? Why? Under what conditions? How often? or To what degree? When adverbs modify adjectives or other adverbs, they usually qualify or intensify the meaning of the word they modify. (See 40e.)
Adjectives are often used incorrectly in place of adverbs in casual or nonstandard speech.
