In passive voice, the writer does not specify who is acting: "The report was written yesterday." It could have been written by the secretary, George Bush, or the manager--we don't know.
The sentence is still in passive voice if the writer specifies the actor later in the sentence: "The report was written yesterday by the manager."
Passive and Active Voice
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Active vs. Passive Voice
Active Voice, Passive Voice
Active vs. Passive Voice Lesson Plan: Teach Students or Yourself When and How to Use Active and Passive Voice When Writing. Learning When to Use Passive and Active Voice Will Improve the Quality of y
Grammar Girl :: Active Voice Versus Passive Voice
Active and Passive Voice
Voice:* Active and Passive
An English-Zone.Com Chart: Active and Passive Voice