The document defines sentences and their types. It states that a sentence is a group of words that expresses a complete thought with a subject and predicate. A sentence begins with a capital letter and ends with appropriate punctuation. It distinguishes sentences from sentence fragments, which do not express complete thoughts. It then describes the five types of sentences: declarative, interrogative, imperative, exclamatory, and optative. Declarative sentences make statements, interrogative sentences ask questions, imperative sentences give commands, exclamatory sentences express strong feelings, and optative sentences express wishes.