Here's why:
* Adverbs modify verbs, adjectives, and other adverbs. They tell us *how*, *when*, *where*, *to what extent*, or *why* something happens.
* Nouns are words that name people, places, things, or ideas. They are not used to modify verbs.
Examples of adverbs modifying verbs:
* He ran quickly. (How did he run?)
* She will arrive tomorrow. (When will she arrive?)
* We walked outside. (Where did they walk?)
* They are completely happy. (To what extent are they happy?)
* He ate because he was hungry. (Why did he eat?)
Let me know if you'd like more examples or have any other grammar questions!