Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
* In the sentence "She closed the door gently," "gently" tells us *how* she closed the door.
Example:
* Adjective: The gentle breeze rustled the leaves. ("Gentle" describes the breeze)
* Adverb: She spoke gently to the child. ("Gently" modifies the verb "spoke")