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 extra information to a main clause and often begins with a subordinating conjunction like 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 help to create complex sentences by providing additional context or details. They can function as adjectives, adverbs, or nouns within a sentence. For instance, in "The book that you lent me was fascinating," the clause "that you lent me" describes the noun book.