Here's how to tell the difference:
Pronoun:
* Replaces a noun or noun phrase: "I saw the dog, and that was a good boy." (That replaces "the dog")
* Acts as the subject or object of a verb: "That is my favorite color." (That is the subject)
* Can be used as an adjective: "That book is interesting." (That modifies "book")
Adverb:
* Modifies a verb, adjective, or another adverb: "He arrived that quickly." (That modifies "quickly")
* Indicates degree or extent: "I like that much." (That indicates the degree of liking)
Example:
* Pronoun: "I saw the dog, and that was a good boy."
* Adverb: "I like that much."
To determine if "that" is an adverb or pronoun, look at its function in the sentence and how it's being used.