Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, or *to what extent* something is done.
* Adjectives describe nouns or pronouns.
In the sentence "She sang loudly," "loudly" tells us *how* she sang. It modifies the verb "sang," making it an adverb.
Example:
* Adjective: The music was loud. (Describes the noun "music")
* Adverb: She sang loudly. (Modifies the verb "sang")