Here's why:
* Adjective clauses modify nouns or pronouns, providing more information about them. They answer the question "which one?" or "what kind?"
* Adverb clauses modify verbs, adjectives, or other adverbs, providing information about time, place, manner, cause, etc. They answer questions like "when?", "where?", "how?", "why?".
Relative pronouns (who, whom, whose, which, that) connect a relative clause to the noun or pronoun it modifies. They act as the subject or object of the adjective clause.
Example:
* The book that I read last night was very interesting. (that introduces an adjective clause that modifies the noun "book")
In contrast, adverb clauses are typically introduced by subordinating conjunctions:
* When I finish my work, I will go to the movies. (when introduces an adverb clause that modifies the verb "will go")
Key takeaway: Relative pronouns always introduce adjective clauses, never adverb clauses.