A noun clause is a group of words that functions as a noun and typically includes a subject and verb. Noun clauses can be used as subjects, subject complements, objects, or objects of prepositions. They often follow verbs and adjectives that express mental activities or opinions and can be used to include questions in statements or report what someone said or asked. There are three types of noun clauses: those introduced by "that", those with wh- words, and those with "if/whether".