When a Spanish ser-passive does name the doer of the action, it does so with the preposition por: El Quijote fue escrito por Cervantes. La carta ha sido firmada por el ministro. El edificio fue diseñado por Gaudí. This is the agent phrase, and it lines up almost perfectly with English "by" in passive sentences. But naming the agent is far from automatic in Spanish — most Spanish passives leave the agent unmentioned — and the preposition por has so many other uses that confusing the agent-por with cause-por is one of the classic learner traps.
This page covers when the agent phrase is required, when it is optional, when it is omitted altogether, how it agrees in form, and how to keep agent-por (the doer) apart from cause-por (the reason) and instrument-por (the means).
The construction at a glance
The agent phrase has three pieces:
- The preposition por (invariable).
- A noun phrase — the agent, the doer of the action.
- Placement: usually at the end of the passive clause, after the participle.
El cuadro fue pintado por Goya en 1814.
The painting was painted by Goya in 1814.
La novela ha sido traducida al español por su mejor amigo.
The novel has been translated into Spanish by his best friend.
El nuevo protocolo será aprobado por la comisión la próxima semana.
The new protocol will be approved by the commission next week.
In each sentence, the subject is the patient (el cuadro, la novela, el nuevo protocolo), the verb is ser + past participle agreeing in gender and number with the subject, and por + a definite agent closes the clause. This is the canonical structure.
When the agent is named: a smaller set than you'd think
Spanish ser-passives are not common in everyday speech to begin with, and ser-passives with a named agent are even rarer. The construction is at home in:
- Formal written prose — history, biography, art criticism, official reports.
- Journalism, especially headline-style or attribution-style sentences: La medida fue aprobada por el Consejo de Ministros.
- Academic writing: Los datos fueron analizados por el equipo de investigación.
- Cultural and historical writing: La catedral fue construida por los maestros canteros del siglo XIII.
El descubrimiento del Pacífico fue realizado por Vasco Núñez de Balboa en 1513.
The discovery of the Pacific was made by Vasco Núñez de Balboa in 1513.
Los resultados del experimento han sido publicados por la revista Nature.
The results of the experiment have been published by the journal Nature.
La sentencia fue dictada por el Tribunal Supremo el pasado martes.
The ruling was issued by the Supreme Court last Tuesday.
Notice that in all these examples the agent is specific, important, and worth naming — Cervantes, Goya, the Supreme Court, the journal Nature. The whole point of choosing the passive construction here is to put the patient (the discovery, the results, the ruling) in subject position while still identifying the responsible agent. If the agent were vague or unimportant, Spanish would reach for the pasiva refleja (Se publicaron los resultados) or the impersonal se (Se dictó la sentencia) instead.
The agent is often omitted
A surprising fact for English speakers: most Spanish ser-passives have no agent phrase at all. The construction is grammatical without one, and writers omit it whenever the agent is obvious, irrelevant, or unknown.
El presidente fue elegido en las elecciones del año pasado.
The president was elected in last year's elections. (the agent — voters — is too obvious to mention)
La novela ha sido traducida a quince idiomas.
The novel has been translated into fifteen languages. (translators not named — irrelevant here)
El edificio fue inaugurado en 1925.
The building was inaugurated in 1925. (who inaugurated it doesn't matter for this sentence)
This is one of the structural reasons why ser-passives feel less common in Spanish than in English: when there is no agent worth naming, Spanish typically prefers an active construction or a pasiva refleja, whereas English happily uses the agentless passive (The building was built in 1925).
Por + agent vs por + cause: the great learner trap
The preposition por is one of the most overloaded words in Spanish. It introduces causes (Lo hizo por dinero — he did it for money), means and routes (Pasamos por Madrid — we went through Madrid), durations and approximate times (por la mañana, por dos horas), exchanges (Te lo cambio por este — I'll trade you for this one), and many other things. The agent of a passive verb is just one of the many roles por fills, and learners regularly mix it up with the others.
The good news is that there is a clean syntactic diagnostic: the agent-por phrase is only available inside a passive construction (ser + past participle, or sometimes estar + participle in result-state readings). Outside of a passive, por + a person almost always means cause, beneficiary, or means.
Compare:
| Use of por | Example | Reading |
|---|---|---|
| Agent (passive) | El libro fue escrito por Cervantes. | By Cervantes (he wrote it) |
| Cause | Lo hizo por dinero. | For money / because of money |
| Beneficiary | Lo hice por ti. | For your sake / because of you |
| Means / instrument | Te lo envío por correo. | By post / via post |
| Route | Pasamos por Madrid. | Through Madrid |
| Approximate time | Llegamos por la noche. | In the evening / around night-time |
El cuadro fue robado por un coleccionista privado.
The painting was stolen by a private collector. (agent — passive construction)
El cuadro fue robado por dinero.
The painting was stolen for money. (cause — same verb, different por)
El cuadro fue robado por un coleccionista privado por dinero.
The painting was stolen by a private collector for money. (both pors — agent first, then cause)
The third sentence is fully grammatical and shows the two por phrases coexisting without conflict. Word order and context disambiguate, and a native ear has no trouble parsing them.
Agent agreement: the participle, not por, does the work
A common student question: does por agree with the agent? No. Por is invariable. What does agree — in gender and number — is the past participle of the passive, and it agrees with the patient subject, not with the agent.
| Subject (patient) | Participle agrees with subject | Agent is independent |
|---|---|---|
| El libro | fue escrito | por Cervantes. |
| La novela | fue escrita | por Cervantes. |
| Los libros | fueron escritos | por Cervantes. |
| Las novelas | fueron escritas | por Cervantes. |
The participle changes (escrito / escrita / escritos / escritas) to track the subject. Por Cervantes never changes. This is the opposite of what some learners expect: it is tempting to think "the agent is the real doer, so it should drive agreement," but Spanish grammar treats the patient as the grammatical subject of the passive and agreement follows the subject.
Las cartas fueron firmadas por el ministro y enviadas por el secretario.
The letters were signed by the minister and sent by the secretary.
El proyecto y la propuesta fueron rechazados por la junta directiva.
The project and the proposal were rejected by the board of directors. (compound subject → masculine plural participle)
Choosing the agent NP: definite, named, and weighty
Agent phrases work best when the agent is a definite, specific, named entity. Por Cervantes is excellent. Por el ministro is fine. Por la empresa is fine. Por un hombre (an unnamed man) is awkward unless context provides identity, and por alguien (by someone) borders on ungrammatical-feeling — Spanish would prefer the active alguien escribió la carta or the impersonal se se escribió la carta.
El informe fue redactado por la consultora McKinsey.
The report was drafted by the consulting firm McKinsey. (definite, named — natural)
❓ La carta fue escrita por alguien que no conozco.
The letter was written by someone I don't know. (grammatical but stilted — the active 'la escribió alguien que no conozco' is more natural)
This restriction is one more reason Spanish ser-passives feel less common than English ones: the construction prefers a named, weighty agent, and when the agent is vague the language defaults to other strategies.
Agent-por with other passive constructions
The classic site of agent-por is the periphrastic passive (ser + past participle), but it also appears with two related constructions, both of which are more formal still.
Estar + participle (resultative)
When estar + past participle expresses a result state (not an ongoing action), an agent phrase with por is occasionally possible — though uncommon and stylistically marked.
El cuadro está firmado por el propio artista.
The painting is signed by the artist himself. (the state of being signed, with the agent named)
El documento está sellado por el notario.
The document is stamped by the notary.
In each case there is a result state (signed, stamped) and the agent of the original action is mentioned. This is less common than the ser-passive version (fue firmado por) and tends to occur in formal/legal registers.
Compound passive with haber sido
The compound passive (ha sido / había sido + participle) takes agent-por in exactly the same way as the simple passive.
La sentencia ha sido confirmada por el Tribunal Supremo.
The ruling has been confirmed by the Supreme Court.
El acuerdo había sido firmado por ambas partes antes del incidente.
The agreement had been signed by both parties before the incident.
Agent-por in formal Spanish: a few worked examples
To get a feel for how agent-por sounds in real formal prose, here are a few examples drawn from the registers where it lives.
La Constitución de 1978 fue aprobada por las Cortes y refrendada por el pueblo español.
The 1978 Constitution was approved by Parliament and ratified by the Spanish people. (historical / civic register)
Las obras fueron suspendidas por la dirección de patrimonio hasta nuevo aviso.
The works were suspended by the heritage department until further notice. (administrative)
El premio Cervantes fue concedido en 2023 por unanimidad por el jurado.
The Cervantes Prize was awarded in 2023 unanimously by the jury. (cultural journalism)
In all three, the agent is institutional and the construction has an air of weight and finality. This is the natural habitat of agent-por.
Common Mistakes
❌ La carta fue escrita para Cervantes.
Incorrect — para suggests the addressee or recipient, not the writer.
✅ La carta fue escrita por Cervantes.
The letter was written by Cervantes.
Confusing por (agent) with para (recipient/purpose). La carta para Cervantes is the letter addressed to Cervantes; la carta por Cervantes is the letter written by Cervantes. See Choosing: por vs para for the broader contrast.
❌ Lo hizo por amor por su madre.
Stylistically odd — two pors with the same role.
✅ Lo hizo por amor a su madre.
He did it out of love for his mother.
Misanalyzing a por phrase as an agent when it is in fact a cause. Por amor is "out of love" (cause), not "by love" (agent). Cause-por takes a + recipient for the beneficiary, not another por.
❌ El libro fue escrito por alguien.
Grammatical but unnatural — vague agent should not appear with named-agent passive.
✅ Alguien escribió el libro. / Se escribió el libro hace siglos.
Someone wrote the book. / The book was written centuries ago.
Using the ser-passive with an indefinite, anonymous agent. Spanish prefers to switch to the active voice or the pasiva refleja when the agent is vague. The ser-passive really wants a definite, named agent.
❌ Las cartas fueron firmada por el ministro.
Incorrect — participle must agree with subject las cartas (feminine plural).
✅ Las cartas fueron firmadas por el ministro.
The letters were signed by the minister.
Forgetting that the past participle in the ser-passive agrees with the subject, not with the agent. Por el ministro is invariable; the participle changes.
❌ El paquete fue enviado por correo por mi madre.
Grammatical but stylistically awkward — two pors in a row read clumsily.
✅ Mi madre me envió el paquete por correo. / El paquete fue enviado por mi madre por correo.
My mother sent me the package by post.
Stacking agent-por and means-por in adjacent position. Spanish allows both, but careful writers reorder or restructure to avoid the visual repetition. Switching to the active voice often produces a cleaner sentence.
Key Takeaways
- Form: in a ser-passive, the agent is introduced by por
- noun phrase. Por is invariable.
- Agreement: the past participle agrees with the patient subject, not with the agent.
- Agent is often omitted: Spanish ser-passives without an agent are common and natural. When the agent is unknown or irrelevant, omit it; if it is vague, switch to pasiva refleja or active voice.
- Disambiguating por: inside a passive structure, por
- an agentive noun is the agent. Outside a passive, or with an abstract or means-of-transport noun, por is doing one of its many other jobs (cause, route, means).
- Register: the named-agent ser-passive is at home in formal writing — history, journalism, law, academic prose. Everyday peninsular Spanish reaches for it sparingly.
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Start learning Spanish→Related Topics
- Pasiva con ser: el libro fue escritoB1 — The full ser-passive: ser in any tense + past participle agreeing with the subject + optional por + agent. Register: formal, written, journalistic.
- Pasiva refleja con se: se venden casasB1 — The se + 3rd-person construction where the verb agrees with the patient — the workhorse passive of everyday Spanish, far more common than the ser-passive in signs, ads, recipes, and journalism.
- Restricciones de la pasiva con serB2 — Why the ser-passive is much less freely available in Spanish than the English passive is in English — the verbs that reject it, the aspects that block it, and what Spanish reaches for instead.
- Por para causa: '¿por qué?'A2 — Spanish 'por' answers 'why?' — it marks cause, motive, reason, and the agent of passive voice. It contrasts with 'para' (purpose / goal), and the difference between 'because of' and 'in order to' is one of the longest-running learner headaches in Spanish.
- Cómo elegir entre por y paraA2 — The canonical Spanish preposition decision. POR points backwards (cause, exchange, route, duration, agent). PARA points forwards (purpose, destination, deadline, recipient, opinion). Memory device, every common use organised by category, peninsular Spanish's distinctive a por construction, and the minimal pairs that train the instinct.