Using “By…” (The Agent)
Learn what the agent is, when it is worth naming with by, when it is better left out, and where it belongs in a passive sentence.
Naming the performer
The agent names who or what performed the action.
The agent is the performer of the action in a passive sentence. It is introduced with by and placed at the end of the sentence, but including it is optional.
In The novel was written by a famous author., by a famous author names the agent, telling the reader who wrote the novel. In The novel was written in 1920., the agent is left out entirely, since the sentence is focused on the date instead.
This lesson covers when to include the agent, when to leave it out, and where it belongs in the sentence.
Before you begin
This is one of ten topics on the passive voice.
This lesson focuses on the agent specifically. The passive voice as a whole includes nine other topics, each covered in its own lesson.
If you would like an overview of the passive voice in general, begin with the Passive Voice page.
The central idea
Naming the agent is a choice, not a requirement.
Compare a passive sentence with the agent named to one without it.
01
Agent named
The novel was written by a famous author.
02
Agent omitted
The novel was written in 1920.
The useful question is: does naming the performer add anything useful?
Both sentences are complete and correct. The choice to name the agent, or to leave it out, depends entirely on what information the sentence needs to communicate.
When to include the agent
Include the agent when it adds real information.
The performer is new or important information
The award was presented by the mayor.
The performer is surprising or notable
The painting was actually completed by a twelve-year-old.
The context requires precision
The decision was approved by the board of directors.
Formal or legal writing often needs the performer clearly identified.
When to omit the agent
Leave the agent out when it adds nothing useful.
The performer is unknown
The window was broken overnight.
The performer is obvious from context
The patient was treated in the emergency room.
It is generally understood that medical staff performed the treatment.
The performer is a vague or generic group
English is spoken all over the world.
Naming by people here would add nothing to the sentence.
Where the agent belongs
The by phrase comes at the very end of the sentence.
The bridge was completed ahead of schedule by the construction crew.
The agent follows any other information about the action, such as time or manner, and comes last in the sentence.
A note on other prepositions
Not every passive-looking phrase uses by.
made of → The table is made of oak.
covered with / in → The hills were covered with snow.
known for → The town is known for its cheese.
married to → She is married to a musician.
These describe a state, not a true agent
These fixed expressions describe a material, condition or relationship rather than naming the performer of an action, so they take a different preposition than the true agent structure.
A useful decision guide
Ask whether naming the performer helps the reader.
01
Does naming the performer add new or important information?
If so, include the agent with by.
02
Is the performer unknown, obvious, or a vague group?
If so, the agent can usually be left out.
03
Where does the agent sit in the sentence?
It comes after any other information, at the very end.
04
Is this a fixed expression describing a state, rather than an action?
Expressions like made of or known for use a different preposition, not by.
Common mistakes
The choice of preposition and the position of the agent both matter.
By, not from or with, introduces a true agent.
Not: The novel was written from a famous author.
Correct: The novel was written by a famous author.
A vague agent often adds nothing to the sentence.
Weak: The cake was baked by someone.
Stronger: The cake was baked.
If the performer cannot be named specifically, leaving the agent out is often clearer than naming it vaguely.
The agent belongs at the end, not in the middle of the sentence.
Not: The bridge was by the crew completed ahead of schedule.
Correct: The bridge was completed ahead of schedule by the crew.
Check your understanding
Should the agent be included in each sentence?
Choose the correct answer for each scenario.
A famous chef personally cooked a meal at a small local restaurant. This is surprising and worth mentioning.
✅ Exactly! A surprising or notable performer is worth naming.
❎ Not quite. Does this performer add genuinely interesting information?
Would readers be interested in who performed this action?
Nobody knows who left the note on the desk.
✅ Exactly! An unknown performer cannot usefully be named.
❎ Not quite. Is the performer known here?
Can the performer even be named specifically here?
The bridge was designed __________ a well-known engineer.
✅ Exactly! A true agent is introduced with “by”.
❎ Not quite. “By” is used to name the performer of a passive action.
What preposition names the performer of an action?
Choose the best explanation
Select the sentence that explains the mistake correctly.
Why might “The cake was baked by someone.” be considered weak writing?
✅ Exactly! A vague agent often adds little, since it doesn’t identify anyone in particular.
❎ Not quite. This is more a style consideration than a grammar error.
Why is “The table is made by oak.” not correct?
✅ Exactly! “Made of” is a fixed expression describing material, not a true passive agent.
❎ Not quite. Think about whether “oak” performed an action, or simply describes what the table is made from.
Quick review
Name the agent only when it adds something useful.
The agent names the performer of the action in a passive sentence, introduced with by.
Include the agent when it is new, important, or surprising information; omit it when the performer is unknown, obvious, or a vague group.
The agent belongs at the end of the sentence, after any other information about the action.
Fixed expressions such as made of, known for and married to describe a state, not a true agent, and use a different preposition.
Remember
Ask whether naming the performer earns its place in the sentence.
If the agent adds real information, name it with by at the end of the sentence. If not, the sentence is often stronger without it.