Here's why:
* Adverbs describe how, when, where, or to what extent an action is performed.
* Verbs express actions or states of being.
Examples:
* Quickly (adverb) ran (verb) - describes how the action was performed
* Yesterday (adverb) she (pronoun) walked (verb) - describes when the action occurred
* Loudly (adverb) he (pronoun) sang (verb) - describes how the action was performed
* Very (adverb) tired (adjective) - modifies the adjective "tired" which describes the state of being
Let me know if you'd like more examples or explanation!