The use of verbs in sentences where the subject is acted upon by the verb instead of doing the action, which is the normal case.

For example:

Active voice: Arthur sought the Holy Grail.

Passive voice: The Holy Grail was sought by Arthur.

The normal giveaway for passive voice is the use of temporal adverbs and clauses such as "was", "has been", "would have been", etc. before the verb.

Passive voice is considered acceptable for research papers, because of its impersonal feel. Excessive use of passive voice in prose, however, is considered bad form, because it can result in confusing sentences and can make prose seem dull and uninvolved.

Use of passive voice is the most common form of grammatical "error" flagged by most modern grammar-checkers.

The passive is a way of changing the focus of roles in a sentence. If Galahad sought the Grail and The Grail was sought by Galahad, although they both refer to the same situation, the first is talking about Galahad and tells what he did, while the second is talking about the Grail and what happens to it. It is a change of topic.

It's different from emphasis in that you can emphasize the direct object without turning it into the passive - Galahad sought the Grail (not the spear) - and can even bring the object forward - The Grail, Galahad sought (while Lancelot sought the spear). And conversely, a passive sentence can have emphasis on another element, such as the subject - The Grail was sought by Galahad (not by Lancelot).

In the active voice sentence Galahad sought the Grail, Galahad is the subject and the Grail is the (direct) object. In its passive transformation The Grail was sought by Galahad, the Grail is now the subject and Galahad is referred to as the agent. In English the agent is marked with the preposition by, and can be omitted - For years the Grail was sought.

The omissibility of the agent means the passive is suitable for expressing situations where the agent is unknown, or not important or relevant: as in The window was broken when I got in this morning, or This book was published in 1962.

The subject commands the agreement on the verb, singular or plural: The Grail was sought by all the knights and The maidens were ravished by Galahad's evil twin brother. So in the passive the object moves into verb-commanding subject position.

The English passive is formed with a part of to be followed by the -en or -ed form of the verb, the past participle, the same part that is used after have to form the active perfect tense: The vase was broken by me is the passive of I broke the vase but uses the verb-form of I have broken the vase. Similar constructions are used in other modern European languages.

In Latin however the passive was a distinct conjugation with its own endings. In general they contain an R but the exact form has to be memorized:

    voco     I call        -->    vocor    I am called
    vocamus  we call       -->    vocamur  we are called
    vocat    s/he calls    -->    vocatur  s/he is called
    vocant   they call     -->    vocantur they are called
And so on through other tenses. Greek also had endings like this, and also for a third voice, the middle voice, denoting roughly a subject acting on or for itself.

In ergative languages you sometimes get a similar transformation called the antipassive, which raises a marked agent into unmarked subject position.

The main reason the passive has a bad reputation in elementary primers of style is that it is overused in officialese: Passengers are requested not to cross the line. The reason that this is tempting to officials is that the subject need not be expressed: The management request passengers not to cross the line unnecessarily personalizes who's doing the requesting. And the reason that's tempting to poor stylists is that it seems to sound more formal. If you remember that the normal and default is the active and the passive is to be used for a refocusing of the topic, you can use it to your heart's content.

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