The sentence is above all a communicative unit; therefore, the primary classification of sentences is based on the communicative principle, traditionally defined as “the purpose of communication”. According to the purpose of communication, sentences are subdivided into declarative, interrogative and imperative. Declarative sentences are traditionally defined as those expressing statements, either affirmative or negative, e.g.: He (didn’t) shut the window. Imperative sentences express inducements of various kinds (orders or requests); they may also be either affirmative or negative, e.g.: (Don’t) Shut the window, please. Interrogative sentences express questions, or requests for information, e.g.: Did he shut the window?