For example, in the sentence:
the verb "ate" is in the active voice. However, in the sentence:
the verbal phrase "was eaten" is passive.
In the sentence:
the verb "killed" is in the active voice, and the doer of the action is the "hunter". A passive version of the sentence is:
where the verbal phrase "was killed" is followed by the word "by" and then by the doer "hunter".
In a transformation from an active-voice clause to an equivalent passive-voice construction, the subject and the direct object switch grammatical roles. The direct object gets promoted to subject, and the subject demoted to an (optional) complement. In the first example above, the mouse serves as the direct object in the active-voice version, but becomes the subject in the passive version. The subject of the active-voice version, the cat, becomes part of a prepositional phrase in the passive version of the sentence, and can be left out entirely.