Here's why:
* Verbs describe actions or states of being. "Is" is the present tense form of the verb "to be." It describes a state of being.
* Adverbs modify verbs, adjectives, or other adverbs. They answer questions like "how?", "when?", "where?", or "to what extent?"
Example:
* Verb: The cat is sleeping.
* Adverb: The cat is sleeping quietly.
In the example, "quietly" modifies the verb "sleeping" and is an adverb. "Is" is the verb and describes the cat's state of being.