The document describes a machine that converts sentences from active to passive voice by following some rules:
1) The verb "to be" is used in the same tense as the main verb in the active sentence and is followed by the past participle of that verb.
2) If the agent (person performing the action) is mentioned, it is introduced with the preposition "by".
3) When the object of the active sentence becomes the subject in the passive, its form may change from singular to plural.
4) Modal verbs are followed by the infinitive form of the verb "to be".