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 B1-e.)

Adjectives are often used incorrectly in place of adverbs in casual or nonstandard speech.

The adverb perfectly modifies the verb worked out; the adverbs smoothly and efficiently modify the verb runs.

Only adverbs can modify adjectives or other adverbs. Really intensifies the meaning of the adjective slim.