Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, or *to what extent* something happens.
* Verbs describe actions or states of being.
* Adjectives describe nouns.
In a sentence, "diligently" would answer questions like:
* How did they work? (They worked diligently.)
Let me know if you'd like more examples!