There are four main types of sentences defined by their function: declarative sentences make statements, interrogative sentences ask questions, imperative sentences give commands or requests, and exclamatory sentences express strong feelings. Declarative sentences typically have a subject followed by a predicate and end with a period. Interrogative sentences can be yes/no questions, alternative questions using "or", wh- questions starting with "what" or "where", or tag questions ending with a tag. Imperative sentences give commands and always have an understood "you" as the subject, ending with a period. Exclamatory sentences convey emotion through exclamation points or periods.