Noun clauses function as nouns and can serve different grammatical purposes within a sentence. They typically begin with words like that, which, who, when, why, how or if/whether. That-clauses use the word "that" to introduce the clause, while wh-word clauses use interrogative words like who, what, when. If/whether-clauses are dependent clauses formed from yes/no questions using if or whether. Noun clauses can serve as subjects, objects, complements, and appositives within sentences.