Here's why:
* Adjectives describe nouns: "Lovely" describes a noun, such as "day," "flower," or "song."
* Adverbs modify verbs, adjectives, or other adverbs: "Lovely" doesn't describe how an action is done (verb) or modify another descriptor.
Examples:
* Adjective: "It was a lovely day." ("lovely" describes "day")
* Adverb: "She sang beautifully." ("beautifully" describes how she sang)