A dependent clause cannot exist alone. It must be attached to a main clause. Dependent adverbial clauses contain an introductory word (e.g. 'Before' the show began, the performers were introduced to the audience). Dependent adjectival clauses contain an introductory relative pronoun (e.g. He bought a machine 'which' could carry out a variety of operations). Full dependent clauses, like main clauses, must contain both a subject and a main verb. In the case of adjectival clauses, the relative pronoun itself constitutes the subject.