The document discusses active and passive constructions in grammar. In an active construction, the subject performs the action of the verb. In a passive construction, the subject has the action performed on it by an agent introduced by "by". Only sentences with a direct object can be changed to the passive voice. The passive is formed using a form of "to be" plus the past participle of the main verb, optionally followed by "by" and the agent. Examples are given for the present, past, and future in both active and passive constructions.