Adverbs usually modify verbs (and can modify adjectives and even other adverbs) and answer the questions:
cf. principle 45.
MBG writes: “The line of distinction between adverbs and other forms of speech can often be difficult to draw. For example, is μόνον the accusative neuter of μόνος functioning adverbially, or has it become set as an adverb? If not μόνον, then how about πρῶτον or ὕστερον?” p. 242