Here's why:
* Adjective clauses describe or modify nouns. They answer the questions "which one?" or "what kind?".
* Noun clauses function as nouns within a sentence. They can act as the subject, object, or complement of a verb.
Relative pronouns (like *who, whom, whose, which, that*) connect these adjective clauses to the noun they modify.
Example:
* The book *that* I read yesterday was very interesting.
* "that I read yesterday" is the adjective clause modifying the noun "book."
* "that" is the relative pronoun.
Important Note: While relative pronouns don't introduce noun clauses directly, they can be used within noun clauses.
Example:
* I know *who* stole the cookies.
* "*who stole the cookies*" is a noun clause functioning as the object of the verb "know."
* "*who*" is the relative pronoun within the noun clause.
So, while not introducing noun clauses directly, relative pronouns play a crucial role in forming adjective clauses that modify nouns within those noun clauses.