Here's why:
* Pronoun: A pronoun replaces a noun or noun phrase to avoid repetition. "It" often stands in for a previously mentioned noun.
* Example: "The dog was barking. It was very loud." ("It" replaces "the dog").
* Adverb: An adverb modifies a verb, adjective, or another adverb. It describes how, when, where, or to what extent something happens.
Let me know if you have any more grammar questions!