Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. "Loudly" describes how someone speaks or makes a sound, which is an action (verb).
* Adverbs often end in "-ly". While not all words ending in "-ly" are adverbs, it's a common indicator.
For example:
* "The dog barked loudly." (modifies the verb "barked")
* "She sang very loudly." (modifies the adverb "loudly")