The document discusses four types of passive voice:
1. Regular passive uses forms of "to be" plus the past participle. It is used to change the focus or when the agent is unknown.
2. Two-object passive involves indirect objects becoming subjects.
3. Impersonal passive uses "it" as the subject and forms of "to be" plus the past participle. It removes an explicit agent.
4. Causative passive uses "have" or "get" instead of "to be" to indicate something was had or got done by a professional.