A subordinate clause is also called a dependent clause. A dependent clause cannot stand by itself as a sentence. It must attach to an independent clause to form what is called a complex sentence.
Dependent clauses may act in three ways: (1) as nouns, (2) as adjectives, and (3) as adverbs. Study these three examples of dependent clauses.
That she won the race did not surprise us. (noun clause acting as the subject of the sentence)
He left the party because he was tired. (adverbial clause modifying the verb left)
He enjoyed the party, which lasted all night. (adjectival clause modifying the noun party)