The passive voice uses forms of the verb "to be" plus the past participle of the main verb. It is used when the recipient of the action is more important than the doer, the doer is unknown or unimportant, or the doer is too obvious to mention. To form the passive, the object of the active sentence becomes the subject of the passive sentence, the main verb is replaced by "to be", and the original subject becomes an optional agent complement introduced by "by".