A noun clause is a dependent clause that functions as a noun within a sentence. Noun clauses contain a subject and verb, but cannot stand alone as they are not complete thoughts. They must be paired with an independent clause. Noun clauses can serve as the subject, direct object, indirect object, or object of a preposition within a sentence. They are generally introduced by words like what, that, why, and how. The subjunctive case is sometimes used in noun clauses to express uncertainty, such as after verbs like require and advise.