Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They describe how, when, where, or to what extent something happens.
* Verbs express actions or states of being.
Example:
* Suddenly, the lights went out. (Here, "suddenly" modifies the verb "went out" by telling us when the action happened.)
While "suddenly" itself is not a verb, you can use it with a verb to express a sudden action:
* The car stopped suddenly.
* The child burst into tears suddenly.