Passive voice is a grammatical voice common in many of the world's languages. Passive is used in a clause whose subject expresses the theme orpatient of the main verb. That is, the subject undergoes an action or has its state changed. A sentence whose theme is marked as grammatical subject is called a passive sentence. In contrast, a sentence in which the subject has the agent role is called an active sentence, and its verb is expressed in active voice Many languages have both an active and a passive voice; this allows for greater flexibility in sentence construction, as either the semantic agent or patient may take the syntactic role of subject.
Transforming an active verb into a passive verb is a valence decreasing process ("detransitivizing process"), because it transforms transsitive verb intointranssitive verb .
The use of passive voice allows speakers to organize stretches of discourse by placing figures other than the agent in subject position. This may be used to foreground the patient, recipient, or other thematik rool Passive voice may also be useful when the semantic patient is the topik of on-going discussion.The passive voice may also be used to avoid specifying the agent of an action.
example:
The metropolis has been scorched by the dragon's fiery breath.
When her house was invaded, Penelope had to think of ways to delay her remarriage.
http://www.unc.edu/depts/wcweb/handouts/passivevoice.html
