When a sentence goes passive — the house was built, the book was written — the thing acted upon becomes the subject, and that raises the obvious question: built by whom? English answers with the little word by: built *by a famous architect. Czech answers with no word at all. It takes the doer — the *agent — and drops it into the bare instrumental case. There is no preposition; the ending alone carries the entire meaning of English by. This is the same seventh case you already use for means and instruments, extended to its most natural relative: the one who did it is treated as the "means" by which the action came about.
English "by + agent" = bare instrumental
Set the two languages side by side. English bolts a by-phrase onto the verb: destroyed by fire, written by Čapek. Czech has no "by" to bolt on, so it lets the case do the work: zničen požárem, napsána Karlem Čapkem. Learn this equation and most passive sentences fall into place.
Dům byl postaven známým architektem.
The house was built by a famous architect. (známým architektem — bare instrumental agent; no word for 'by')
Kniha byla napsána Karlem Čapkem.
The book was written by Karel Čapek. (Karlem Čapkem — both first and surname in the instrumental)
Tato budova byla zničena požárem v roce 1945.
This building was destroyed by fire in 1945. (požárem — even a non-human cause takes the bare instrumental)
Where the agent lives: only the participial passive
The bare-instrumental agent is available in one specific construction: the participial passive, built with být plus a passive participle (the -n- / -t- form). Czech has a second passive, the reflexive se-passive (kniha se napsala in a generic sense), and that one cannot name an agent at all — there is no slot for it. So whenever you actually want to say who did it, you must use the být + participle passive. The structure is: subject — být — passive participle — agent (instrumental).
Smlouva byla podepsána oběma stranami.
The contract was signed by both parties. (oběma stranami — instrumental plural agent)
Pacient byl operován zkušeným chirurgem.
The patient was operated on by an experienced surgeon. (zkušeným chirurgem — adjective + noun, both instrumental)
Rozhodnutí bylo schváleno celým výborem.
The decision was approved by the whole committee. (celým výborem — instrumental, the collective agent)
The participle agrees in gender and number with the subject, not with the agent: dům byl postaven (masc. sg.), kniha byla napsána (fem. sg.), okna byla rozbita (neut. pl.). The agent just sits in the instrumental regardless. For the full mechanics of forming and agreeing the participle, see the participial passive page.
The agent endings
The agent takes ordinary instrumental endings; nothing special happens because it is an agent. The masculine and neuter singular ending is -em, the feminine singular -ou, and proper names follow the same pattern. Here is a quick reference for the kinds of nouns that most often appear as agents:
| Nominative | Instrumental (agent) | English "by …" |
|---|---|---|
| architekt | architektem | by an architect |
| autor | autorem | by an author |
| autorka | autorkou | by a (female) author |
| Karel Čapek | Karlem Čapkem | by Karel Čapek |
| vláda | vládou | by the government |
| policie | policií | by the police |
| oheň / požár | ohněm / požárem | by fire |
Zákon byl schválen vládou minulý týden.
The law was approved by the government last week. (vládou — feminine instrumental -ou)
Zloděj byl zadržen policií přímo na místě.
The thief was detained by the police right on the spot. (policií — instrumental of policie)
Obraz byl namalován neznámou holandskou malířkou.
The painting was painted by an unknown Dutch (female) painter. (neznámou holandskou malířkou — three feminine instrumentals in a row)
Agent versus source: instrumental versus od + genitive
There is a subtle line worth drawing, because od + genitive also translates as English from / by and tempts the learner. The canonical agent of the action — the one performing the verb — is the bare instrumental: napsán autorem (written by the author). But od + genitive marks a source or originator, often with verbs of receiving, sending, and the like, and is not the way to mark the doer of a passive verb. You receive a letter od kamaráda (from a friend), but a letter is written kamarádem (by a friend) when you passivise psát.
Dopis byl napsán mým kamarádem.
The letter was written by my friend. (kamarádem — instrumental, the agent who wrote it)
Dostal jsem dopis od kamaráda.
I got a letter from a friend. (od kamaráda — genitive, the source/sender, not a passive agent)
When to use the agentive passive at all
A practical word on register, because overusing it is itself a kind of error. The agentive passive is (formal) and at home in news, law, science, and academic prose: byl jmenován, bylo zjištěno, byla podepsána. In everyday (informal) speech, Czech far prefers the active voice with the doer as subject — Ten dům postavil známý architekt ("a famous architect built that house") — or a subjectless plural — Postavili to v roce 1920 ("they built it in 1920"). So reach for the instrumental agent when you genuinely need the passive frame; don't manufacture passives where Czech would simply say it actively. The full decision is laid out on choosing the passive.
Most byl otevřen prezidentem za přítomnosti novinářů.
The bridge was opened by the president in the presence of journalists. (prezidentem — formal, news register)
Ten dům postavil můj děda.
My grandfather built that house. (the everyday active version — no passive, no instrumental needed)
Common Mistakes
Almost every error here comes from importing the English by as a word, or from confusing source with agent.
❌ Dům byl postaven od architekta.
Incorrect — the agent of a passive is the bare instrumental; od + genitive marks a source, not the doer.
✅ Dům byl postaven architektem.
The house was built by an architect.
❌ Kniha byla napsána s Karlem Čapkem.
Incorrect — s + instrumental means 'together with'; the agent takes the bare instrumental, no s.
✅ Kniha byla napsána Karlem Čapkem.
The book was written by Karel Čapek.
❌ Dům byl postaven architekt.
Incorrect — the agent is left in the nominative; it must be in the instrumental.
✅ Dům byl postaven architektem.
The house was built by an architect.
❌ Smlouva se podepsala oběma stranami.
Incorrect — the reflexive se-passive cannot name an agent; use the být + participle passive instead.
✅ Smlouva byla podepsána oběma stranami.
The contract was signed by both parties.
Key Takeaways
- English by + agent becomes a bare instrumental — no preposition: postaven architektem, napsán Čapkem.
- The agent lives only in the být + passive participle construction; the reflexive se-passive has no agent slot.
- Endings are the ordinary instrumental: masc./neut. -em, fem. -ou (autorem, autorkou, vládou, policií).
- Od
- genitive is a source ("from"), not the passive agent — don't substitute it for the doer.
- The agentive passive is (formal); everyday Czech prefers the active voice or a subjectless plural.
Now practice Czech
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Start learning Czech→Related Topics
- Expressing the Agent in the PassiveC1 — Naming who or what did it with the instrumental case (and od + genitive).
- The Participial Passive (být + -n/-t participle)B2 — Forming the periphrastic passive with být and the passive participle.
- Choosing Between the Two PassivesB2 — A decision guide for when to use the reflexive passive (se) versus the participial passive (být + participle) in Czech.
- The Instrumental of MeansA2 — Using the instrumental to express the tool or means by which something is done.
- The Instrumental as Predicate (stal se učitelem)B1 — Why professions, roles, and changed states after být and stát se take the instrumental.