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When to mention or omit the agent using by.
The agent is the person, group, or thing that performs the action expressed by a passive verb.
In passive sentences, the agent can be introduced with by, but it is often omitted when it is unknown, obvious, unimportant, or intentionally not mentioned.
Use by to introduce the agent that performs the action in a passive sentence.
For example, The novel was written by George Orwell and The decision was made by the committee.
The agent is commonly omitted when the identity of the person performing the action is unknown, obvious, irrelevant, or unimportant.
For example, My car was stolen and The road is being repaired.
Omit the agent when the person or thing responsible for the action is not known.
For example, The window was broken during the night.
The agent can be omitted when the context makes it obvious who performed the action.
For example, The suspect was arrested after the attack. The identity of the police officers may be obvious from the context.
The agent can be omitted when the action or result is more important than who performed it.
For example, The new bridge was completed last month focuses on the bridge and its completion rather than the workers.
A speaker or writer may deliberately omit the agent when assigning responsibility is undesirable, unnecessary, or not relevant.
For example, Mistakes were made can avoid explicitly identifying who was responsible.
Agents expressed by general nouns such as people, someone, they, or the public are often omitted when the passive sentence remains clear without them.
For example, English is spoken all over the world is more natural than English is spoken by people all over the world.
Formal and academic writing often omits agents when the process or result is more important than the person performing the action.
For example, The samples were collected and The results were analyzed are common scientific constructions.
Include the agent when identifying who performed the action is important to the meaning, contrast, or purpose of the sentence.
For example, The painting was created by Picasso distinguishes the creator from other possible artists.
The agent performs an action, while an instrument is the means or tool used to perform it.
For example, The window was broken by the boy identifies the agent, while The window was broken with a hammer identifies the instrument.
By commonly introduces an agent, while with commonly introduces an instrument, material, or means.
Compare The cake was made by Sarah with The cake was cut with a knife.
The agent introduced by by can be an organization, institution, government, company, or other collective entity.
For example, The policy was introduced by the government and The software was developed by the company.
By can sometimes introduce a natural force or other cause conceptualized as the source of an event.
For example, The trees were damaged by the storm.
Pronouns used after by normally take object forms.
For example, The message was sent by him and The decision was made by us.
A by-phrase can contain a noun phrase with modifiers and coordination.
For example, The project was approved by the senior management team and The decision was made by John and Sarah.
The by-phrase normally follows the passive verb and its required complements.
For example, The book was written by a well-known author and The problem was solved by the engineering team.
In complex passive sentences, the agent may appear later in the sentence to keep the main information easier to process.
For example, The proposal, which had been discussed for several months, was finally approved by the board.
Technical writing often omits the agent because the procedure or result is more important than the person who performs it.
For example, The solution was heated to 80°C and The samples were then examined under a microscope.
Whether the agent is expressed depends on what information the speaker or writer wants to foreground.
The passive can place the affected person, thing, or process in subject position while moving the agent into a less prominent position.
An agent is especially useful when contrasting who performed similar or different actions.
For example, The first experiment was conducted by Smith, while the second was conducted by Jones.
Omitting the agent can make responsibility less explicit or allow attention to remain on the event or result.
This is common in institutional, political, and formal language where the identity of the responsible party may be backgrounded.
Passive constructions containing reporting or causative patterns may include or omit an agent according to the intended focus.
For example, The decision is believed to have been made by the committee places the agent in a secondary position.
Mention the agent when it adds meaningful information; omit it when it is unknown, predictable, irrelevant, or intentionally backgrounded.
The choice should be based on communicative purpose rather than on a fixed rule that passive sentences should always omit the agent.
Use by for agents and with for instruments, and omit the agent only when doing so is appropriate.
Common mistakes include using by instead of with for instruments, using subject pronouns after by, unnecessarily repeating an obvious agent, and omitting an important agent that changes the meaning of the sentence.