A subordinate clause is also popularly known as the dependent clause. It starts with a subordinate conjunction or a relative pronoun and has both complete set of subject and verb. Though it has a subject and a verb it still does not form a complete thought urging readers ask for more information to understand the thought. The subordinated clause in the statement above is "who had not studied the lessons failed the test". "Who" is the relative pronoun and at the same time the subject while failed is the verb.
