In a Spanish passive sentence, the agent — the person or thing that actually performs the action — is introduced by the preposition por. This matches English by: "written by the author" → escrito *por el autor*.
Unlike English, however, Spanish speakers frequently leave the agent out entirely. When the doer is unknown, unimportant, or obvious from context, the agent disappears and the sentence simply focuses on what happened.
The basic pattern
subject + ser + past participle + por + agent
The agent slot can be a single noun, a proper name, a pronoun, or a longer noun phrase. Whatever fills that slot is treated as the logical "doer" of the verb.
La carta fue escrita por María.
The letter was written by María.
El castillo fue destruido por un terremoto.
The castle was destroyed by an earthquake.
Both a human agent (María) and a non-human force (un terremoto) work equally well. Por does not care whether the doer is animate.
Por, not de
Modern Spanish uses por for the agent in virtually all cases. You may occasionally run into de in older literary texts or with a few verbs of emotion (ser amado de todos), but in Latin American Spanish today, por is the default and safest choice.
When to include the agent
Include the agent when it adds information the reader actually needs:
- The identity of the doer is newsworthy or surprising.
- You are giving credit or attribution (authors, artists, inventors).
- The contrast between possible doers matters.
La teoría fue propuesta por una joven científica mexicana.
The theory was proposed by a young Mexican scientist.
El mural fue pintado por Diego Rivera en 1934.
The mural was painted by Diego Rivera in 1934.
In both of these, the point of the sentence is who did it, so naming the agent is essential.
When to leave the agent out
Leave the agent out when it would be obvious, unknown, or irrelevant. This is extremely common — probably more common than including it.
El acuerdo fue firmado la semana pasada.
The agreement was signed last week.
Los heridos fueron trasladados al hospital.
The wounded were taken to the hospital.
Neither of these needs "by whom". Any reasonable reader fills in the gap: the relevant officials signed the agreement, paramedics moved the injured. Adding por ellos would be clumsy.
Agentless passive vs. passive se
When Spanish drops the agent from a ser-passive, the result often sounds formal or journalistic. For less formal, agent-free statements, speakers very often switch to the passive se instead. Compare:
- El libro fue publicado en 2020. (formal)
- Se publicó el libro en 2020. (neutral, more natural in speech)
Both are correct. The ser-passive just has a more written, elevated register. See Restrictions on the Passive for more on when each option is appropriate.
Longer agent phrases
The agent after por can be quite long, with its own modifiers, relative clauses, or lists. Word order stays the same: everything in that phrase belongs to the agent.
La decisión fue tomada por el comité que se reunió el lunes.
The decision was made by the committee that met on Monday.
El premio fue entregado por el ministro de cultura.
The prize was presented by the minister of culture.
Notice that the participle still agrees only with the subject (la decisión, el premio), never with the agent after por. The agent does not influence agreement at all.
Por vs. other uses of por
Por has many meanings ("through", "because of", "in exchange for", etc.). In a passive construction, though, its role is unambiguous: if you see ser + participle, anything that follows por is the agent. Context rules out the other meanings.
Este método fue inventado por un ingeniero alemán.
This method was invented by a German engineer.
Here there is no risk of reading por un ingeniero alemán as "because of" — passive grammar forces the agent interpretation. Trust the structure.
Related Topics
- Passive with Ser + Past ParticipleB2 — Form the true passive voice in Spanish using ser plus a past participle that agrees with the subject.
- Restrictions on the PassiveB2 — Why the ser-passive is less common in Spanish than in English, and what sentences simply do not work in it.
- Active vs Passive: Which to UseB2 — Decide between active voice, passive se, and ser-passive depending on whether the agent matters and how formal the context is.