Differences Between Relative Pronoun and Subordinating Conjunction

 The main difference between a relative pronoun and a subordinating conjunction is that a relative pronoun replaces the subject of an independent clause, while a subordinating conjunction connects an independent clause to a dependent clause.

Example: She wants a book. The book is inexpensive. -> She wants a book that is inexpensive. The word that replaces the word book in the relative clause, and the clause modifies a noun. (There’s nothing about the verb that makes it unable to stand alone.)

She wants a book. She is bored-> She wants a book because she is bored. Because does not replace any words in the dependent clause. The clause is an adverb clause telling us *why* she wants a book (not describing the book she wants.) Again, nothing about the verb has changed from when we went from an independent to a dependent clause.