Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They often describe how, when, where, or to what extent something happens. They frequently end in "-ly".
* Adjectives describe nouns or pronouns. They tell us what kind, which one, or how many.
For example:
* Slowly (adverb) - modifies the verb "walk": "He walked slowly."
* Beautiful (adjective) - describes the noun "flower": "The flower was beautiful."