Many people don't know what the passive voice is. We're all told to avoid it, but not how to identify it when it sneaks up on us. And it's not always something to avoid.
Active versus Passive
In the active voice, the subject is the actor or agent, and the object is the patient (the recipient of the action). For example:
Raoul baked a cake.
Raoul is both the subject and the actor. The sentence is about Raoul and what he did. The cake is both the object and the patient.
In the passive voice, the subject is the patient and the object is the actor:
A cake was baked by Raoul.
The sentence is about the cake and what happened to it. The cake is the subject of the sentence. Despite being the actor, Raoul is the object of the sentence. The passive voice doesn't require an actor. If I don't want to tell you about Raoul, I can just say A cake was baked.
The passive voice needs a patient—a thing that the actor does something to. This usually requires a transitive verb like throw or love that transfers action. You can throw someone or love someone. In the passive voice, someone can be loved by you or be thrown by you.
When to Use the Passive Voice
The passive voice is good for two things: to avoid talking about the actor, or to create emphasis by moving words around.
If the actor is unknown, or you don't want to reveal the actor, the passive voice lets you omit it:
Stone tools were carved over 2 million years ago. We don't know who carved them.
Mistakes were made. We don't want to admit who made them.
You can emphasize the actor by placing it at the end of the sentence, or make the patient the subject by placing it at the beginning:
He was killed by his own sister. (Emphasis: actor)
The first stone tools were discovered in Tanzania. (Subject: patient)
Don't be afraid to use the passive voice when you need it.
When to Avoid the Passive Voice
The active voice is usually clearer and more concise than the passive. Unless you have a reason to use the passive voice, use the active voice.
The passive voice doesn't work so well with intransitive verbs, which don't transfer action. When someone dies or smiles, they aren't doing it to someone else. There is no passive voice equivalent for Steadman smiled, because without a patient there can be no passive subject: *? was smiled by Steadman.1
Many verbs can be both transitive and intransitive, and sometimes intransitive verbs can be used transitively. In Jane died a thousand deaths, there is technically a patient—death—but it seems strange to form a passive such as *A thousand deaths were died by Jane.2
The passive voice also doesn't work with the verb to be, which is neither transitive nor intransitive. Verbs like be, seem, and become are copular or linking verbs. Rather than creating action, they link the subject and predicate. The sentence Bessie was a cow has no passive equivalent: *A cow was been by Bessie.
Why People Are Confused
Because the passive voice uses the verb to be, other constructions3 are sometimes mistaken4 for it.
Consider this example of the "passive voice" from The Elements of Style by Strunk and White:
There were a great number of leaves lying on the ground.
There is no patient! This sentence can't be construed as passive voice. There were is an example of an expletive construction—wordy, but not passive.
In fact, of four "passive voice" examples Strunk and White provide, three are wrong.
With such poor guidance from such a popular grammar book, it's no wonder people don't know what the passive voice is.
An indirect object can be a patient. Steadman smiled at me can be made passive: I was smiled at by Steadman. But Steadman smiled doesn’t take a direct object, so there is no passive equivalent.
Using indirect objects as patients to intransitive verbs can be an interesting effect. Sentences like The van had been lived in by ghosts or Niedermeyer was passed to by the midfielder sound natural. It doesn’t always work: you wouldn’t say The van was died in by Jane.
The present progressive (is throwing) looks like the passive voice (is thrown), for example.