Here's why:
* Nouns name people, places, things, or ideas (e.g., dog, park, book, happiness).
* Adverbs describe verbs, adjectives, or other adverbs, often telling how, when, where, or to what extent something is done (e.g., happily, quickly, yesterday, very).
In the sentence "She lived happily ever after," "happily" describes how she lived.