The passive voice in English is a grammatical voice whose syntax is marked by a subject followed by a stative verb complemented by a past participle. For example: The enemy was defeated. Caesar was stabbed. In each instance of a passive voice construction, the subject denotes the recipient of the action (the patient) rather than the performer (the agent). The agent may be omitted as evinced in the examples above, or it may be included adjunctively as follows: The enemy was defeated by our troops. Caesar was stabbed by Brutus. Conversely, an active voice construction of the foregoing examples yields the following analogues: Our troops defeated the enemy. Brutus stabbed Caesar. A form of the verbs be or get typically comprises the stative aspect of the English passive voice construction, and the pertinent passive participle is sometimes called a passive verb.
English allows a number of passive constructions that are not possible in many of the other languages with similar passive formation. These include promotion of an indirect object to subject (as in Tom was given a bag) and promotion of the complement of a preposition (as in Sue was operated on, leaving a stranded preposition).
Use of the passive in English varies with writing style and field. It is generally much less used than the active voice but is more prevalent in scientific writing than in other prose. Contemporary style guides discourage excessive use of the passive but appropriate use is generally accepted, for instance where the patient is the topic, the agent is unimportant (and therefore omitted), or the agent is to be highlighted (and therefore placed toward the end).
Identifying the English passive
The passive voice is a specific grammatical construction. The essential components, in English, are a form of the stative verb be (or sometimes get) and the past participle of the verb denoting the action. The agent (the doer of the action) may be specified using a prepositional phrase with the preposition by, but this is optional. It can be used in a number of different grammatical contexts; for instance, in declarative, interrogative, and imperative clauses: "Kennedy was assassinated in 1963." "Mistakes were made." "The window got broken." "Have you ever been kicked by an elephant?" "Don't get killed." "Being attacked by Geoffrey Howe was like being savaged by a dead sheep."
Misuse of the term
Though the passive can be used for the purpose of concealing the agent, this is not a valid way of identifying the passive, and many other grammatical constructions can be used to accomplish this. Not every expression that serves to take focus away from the performer of an action is an instance of passive voice. For instance, "There were mistakes" and "Mistakes occurred" are both in the active voice. Occasionally, authors express recommendations about use of the passive unclearly or misapply the term "passive voice" to include sentences of this type. An example of this incorrect usage can be found in the following extract from an article from The New Yorker about Bernard Madoff (bolding and italics added; bold text indicates the verbs misidentified as passive voice):
The intransitive verbs would end and began are in fact ergative verbs in the active voice. Although the speaker may be using words in a manner that diverts responsibility from him, this is not being accomplished by use of passive voice.
Reasons for using the passive voice
The passive voice can be used without referring to the agent of an action; it may therefore be used when the agent is unknown or unimportant, or the speaker does not wish to mention the agent. Three stores were robbed last night (the identity of the agent may be unknown). A new cancer drug has been discovered (the identity of the agent may be unimportant in the context). Mistakes have been made on this project (the speaker may not wish to identify the agent).
The last sentence illustrates a frequently criticized use of the passive, as the evasion of responsibility by failure to mention the agent (which may even be the speaker themselves).
Nonetheless, the passive voice can be complemented by an element that identifies the agent, usually via a by-phrase that is intended to emphasize the agent. For example: Don't you see? The patient was murdered by his own doctor!