Agent represents the person or a thing that performs the action, subject of an active sentence.
The structure of the passive sentence is:
Passive clause + by + Agent…
In passive sentences, we don’t necessary know who/what the agent is.
But if an agent is explicit, we insert by before him/her/it.
NOTE: We can transform the passive voice into an active voice, by putting the agent in the subject position.
- Hamlet is written by William Shakespeare.
- This beauty salon is run by a woman from New Zealand.
- The house was affected by the earthquake.
The majority of the passive sentences don’t have any agent.
However we use the agent when it is:
- An indefinite noun phrase conveying new information;
- An unexpected inanimate noun;
- The proper name of an artist, inventor, discoverer, innovator.