The document discusses active and passive voice in sentences. In the active voice, the subject performs the action, as in "A dog chased that girl." In the passive voice, the subject receives the action, as in "That girl was chased by a dog." To form the passive voice, an auxiliary verb such as "was" is used along with the past participle of the main verb, as in "was chased." The passive voice can also be used with modal auxiliaries and perfect or progressive tenses.