The majority of passive sentences in English do not include explicit agents to indicate exactly who performed the actions (Shintani, 1979); however, agents exist; and they can be indicated, if need be, by a prepositional phrase beginning with by
a. My car was stolen by someone.
b. A decision has been made by the group.
c. Much tobacco is grown in Eastern Europe by farmers.
d. A new president has been elected by the voters.
e. Both French and English are spoken in Canada by the populace.
Although most passive voice sentences in English do not include agents, there are three narrow instances when speakers and writers tend to express them (Shintani, 1979):
1. The agent is expressed when it is a proper name indicating an artist, an inventor, a discoverer, or an innovator.
a. The Mona Lisa was painted by Leonardo da Vinci.
b. The Americas were discovered by Christopher Columbus.
c. The first computer was built by Charles Babbage.
d. Rubber was first vulcanized by Charles Goodyear.
2. The agent is expressed when it is an indefinite noun phrase conveying new information that the speaker/writer thinks is important enough to mention.
a. These works of art were all produced by a woman.
b. The corner store was robbed by a masked gunman.
3. The agent is expressed when it is an unexpected inanimate noun.
a. Thirteen people were injured by a tornado in Florida.
b. All the lights in this building are controlled by computers.
With few exceptions, passive voice sentences can be recast in the active voice with the agent moved to the subject position with no essential change of meaning. Note these examples:
a. Someone stole my car.
b. The group has made a decision.
c. Farmers grow much tobacco in Eastern Europe.
d. The voters have elected a new president.
e. The populace speaks both French and English in Canada.
f. Leonardo da Vinci painted the Mona Lisa.
g. A woman produced these works of art.
h. A tornado injured thirteen people in Florida.
Summary of Agents in Passive Voice Sentences
1. Agents always exist in passive voice sentences, mostly unexpressed but sometimes expressed by means of a by-phrase.
2. An agent is most frequently expressed when it is
a. the proper name of an artist, inventor, discoverer, innovator;
b. an indefinite noun phrase conveying new information;
c. an unexpected inanimate noun.
3. Passive voice sentences can be recast into the active voice with the agent in the subject position.