If the subject of a clause acts, the clause is in the active voice. If the subject is acted upon, the clause is in the passive voice. For example, I kissed Sheila is in the active voice because the subject (I) acts (kissed) upon the object of the verb (Sheila). I was kissed by Sheila is in the passive voice because the subject is being acted upon.
Voice only applies to verbs that are transitive (i.e., verbs that have direct objects). For example, the intransitive verb wait cannot be passive because passive constructions such as I was waited make no sense. (Many intransitive verbs become transitive when paired with prepositions, however; for example, the phrasal verb wait on is transitive, so it can be passive—e.g., I was waited on by the tall server.)
Forming the passive voice
The passive voice is usually generated by placing a form of the verb to be (i.e., am, are, is, was, were) before the predicate verb’s past participle. So, for example, I own the book becomes The book is owned by me. In informal contexts, to get sometimes replaces to be—for example, I got kissed by Sheila.
Sometimes when the passive voice is used in a dependent clause, the to be verb can be dropped. For example, in the sentence, I bought the book recommended to me by Sheila, the phrase that was is implied between book and the dependent clause recommended to me by Sheila. The same sentence with the dependent clause in the active voice might read, I bought the book Sheila recommended.
When to use passive voice
Some English authorities recommend against using passive voice where it could be avoided. There are three main reasons for this: (1) Passive voice is usually wordier than active voice. (2) Passive voice can be used to omit the agent of an action in order to hide responsibility (e.g., Shots were fired). (3) Passive-voice sentences are harder to follow, requiring a longer processing time in the reader’s mind. The first two points are worth keeping in mind while writing, and the third is probably baseless.
Although active voice is usually better for getting information across, there are times when the passive voice is better, such as when you need to hide an actor’s identity or when the identity of the actor is not important. In other cases, you may wish to shift focus to the thing being acted upon, or you may simply feel the passive voice sounds better.
In any case, ignore the blanket condemnations of the passive voice. Flip through a few of your favorite books, and you’ll see even great authors use the passive voice fairly often. You might notice that even people who rail against the passive voice use it often.