Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* Nouns name people, places, things, or ideas.
In a sentence, "hardly" would typically describe the extent or degree of something.
Example:
* "I hardly ate anything for breakfast." (Here, "hardly" modifies the verb "ate" and tells us the extent to which the speaker ate breakfast.)