Here's why:
* Verbs describe actions or states of being. "Flew" describes the action of moving through the air.
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
For example:
* Verb: The bird flew.
* Adverb: The bird flew quickly. ("quickly" modifies the verb "flew")
Let me know if you have any other grammar questions!