Passive Voice

Usage
The active voice of a verb is used when the subject of the verb refers to the person or thing performing the action described by the verb. In contrast, the passive voice of a verb is used when the subject of the verb refers to the person or thing receiving the action described by the verb. Only a verb which can take an object, i.e. a transitive verb, can be put into the passive voice.

The Passive Voice is more commonly used in English than it is in other European languages such as Spanish or French. As well as being used in everyday English, the passive voice is used extensively in official documents and scientific papers. 

Formation
In the following examples, the verbs in the passive voice are boldtyped:
Active: The boy struck the ball.
Passive: The ball was struck by the boy.
Active: The explorers have found gold.
Passive:Gold has been found by the explorers.

In these examples, the verbs was struck and has been found are in the passive voice. The subjects ball and gold refer to things receiving the actions described by the verbs.

For every tense in the active voice, there is a corresponding tense in the passive voice. In the passive voice, the verb to be acts as an auxiliary. The passive voice tenses of an English verb are formed from the corresponding conjugations of to be, followed by the past participle of the verb.
 

Observe:
The wind is rippling the water > the water is being rippled by the wind
The child's going to open the parcel > The parcel is going to be opened by the child

It should be noted that, when changing the voice of a verb in a sentence while preserving the meaning of the sentence, it is necessary to make sure that the verb agrees with its new subject.
The squirrel ate the nuts > The nuts were eaten by the squirrel
The boys are mowing the lawn > The lawn is being mowed by the boys


Two-object Structures
When a verb in the active voice takes both a direct object and an indirect object (ditransitive constructions), either object can become the subject of the verb when the verb is put into the passive voice, and the meaning of the sentence is preserved. The object which does not become the subject remains as an object. When a verb in the passive voice takes an indirect object, the indirect object is usually preceded by a preposition. However, the preferable and most common construction is using the indirect object as the passive subject:
Active: The guide will show you the museum.
Passive1: You will be shown the museum by the guide.
Passive2: The museum will be shown to you by the guide.

Omission of the Agent
It has been estimated that only 15 to 20 percent of passive sentences mention the agent (in a phrase with by). We do not normally use the by-phrase in the following cases:

1. When the agent is unknown.
My car was stolen. ( I do not know who stole it.)
This table was made in England. (I do not know who made it.)

2. When the agent is obvious or unimportant.
A lot of rice is eaten in Japan. (Obviously by the Japanese.)
Oranges are grown in California and Florida. (Obviously by orange growers.)
An incision is made around the optic nerve and the optic nerve is released. (This example is a good illustration of using the passive to emphasize a procedure or process. It is obvious that a surgeon performs the operation. What is important here is what the surgeon must do, not the surgeon himself.)

3. When we do not want to mention the agent.
The administration admitted that mistakes were made in its handling of the affair.
(The administration does not want to blame anybody, or perhaps does not want to accept responsibility, so the by-phrase is not used.)

4. When the agent has been mentioned previously.
Don Quixote was written by Cervantes. It was finished in 1616.
(It was finished by Cervantes, who has already been mentioned.)

Inclusion of the Agent
The agent with by is usually expressed:

1. When the agent is new information.
This is a true story and was told to me by my mother when I was a little boy.
A: I really like the red paint on your car.  B: Thanks. It was painted by my friend Bob.

2. When the agent is not human.
In a shunt motor, speed is controlled by a rheostat connected in series with the field windings.
Most standardised tests are corrected by a computer.

3. When the agent is well known and should be included because it is important information.
The fountain in Lyon, located in front of Lyon's city hall, was designed by Bartholdi, the designer of the Statue of Liberty.


Practice:
Exercise1, Exercise2, Exercise3, Exercise4, Exercise5.