The good boy behaved well.
In English language, an adverb can be defined as a word that is used to modify a verb, adjective, or another adverb in a sentence. Some examples are slowly, quickly, brightly, sadly, promptly, etc.
Additionally, there are six (6) main types of adverbs and these includes;
I. Adverb of time.
II. Adverb of frequency.
III. Adverb of place.
IV. Adverb of manner.
V. Adverb of reason.
VI. Adverb of intensity.
In this scenario, the adverb "well" is used to modify the action (a verb) of the boy, which is his behavior. Thus, it was used to describe the degree or extent to which the boy's behavior was.
In conclusion, well is an example of an adverb of intensity which describes the degree of an action or something.