A subordinate clause is a group of words that contains a subject and a verb but cannot stand alone as a complete sentence. It adds additional information to a main clause and often begins with a subordinating conjunction, such as because, although, or if. For example, in the sentence "I will go to the park if it stops raining," the clause "if it stops raining" is subordinate. Subordinate clauses can function as adjectives, adverbs, or nouns within a sentence. They help to create complex sentences that convey more detailed ideas. For instance, in "The book that you lent me was fascinating," the clause "that you lent me" describes the noun book.