Modifiers
Almost every word in the sentence is capable of having its meaning described, restricted, emphasized, or toned down in some way—that is, any meaning in the sentence may be characterized or modified.
Heavy automobiles run smoothly.
Heavy both describes and restricts automobiles.
Words which characterize nouns in this way are called adjectives.
Smoothly describes and restricts run.
Words which characterize verbs are called adverbs.
Very heavy automobiles run much more smoothly.
Very emphasizes heavy. A word which thus characterizes an adjective is called an adverb.
More characterizes smoothly. Much characterizes more. A word which characterizes an adverb is called an adverb.