kdo (who) and co (what) are two of the hardest-working words in Czech. They ask questions, they introduce relative clauses, and — once you add the ně- and ni- prefixes — they become the base for someone, something, nobody, nothing and a dozen more. Two things make them tricky for English speakers: they decline through all the cases (English who barely changes, what not at all), and they force a fixed singular agreement that ignores how many people or things you are really talking about. Get those two facts straight and a whole region of Czech grammar opens up.
The declension
kdo and co each run through the full set of cases. They have no gender and no plural — there is just one form per case:
| Case | kdo (who) | co (what) |
|---|---|---|
| Nom. | kdo | co |
| Gen. | koho | čeho |
| Dat. | komu | čemu |
| Acc. | koho | co |
| Loc. (o…) | (o) kom | (o) čem |
| Instr. | kým | čím |
Two patterns to lock in. For kdo, the genitive and accusative are identical — koho — which mirrors how masculine animate nouns work. For co, the soft stem č- appears in every oblique case: čeho, čemu, čem, čím. The English speaker's habit of leaving the question word unchanged (s kdo, o co) is the number-one error here.
kdo and co in questions
In a question, the pronoun takes whatever case the verb or preposition demands — exactly as a noun would. Compare these and notice how the case is dictated by the grammar of the sentence, not by the fact that it is a question:
Koho hledáš?
Who are you looking for? (hledat takes the accusative → koho)
Komu to patří?
Who does this belong to? (patřit takes the dative → komu)
S kým jdeš na večeři?
Who are you going to dinner with? (s takes the instrumental → kým)
O kom to mluvíš?
Who are you talking about? (o takes the locative → kom)
The co side works the same way, with the soft č- stem doing the bending:
Čeho se bojíš?
What are you afraid of? (bát se takes the genitive → čeho)
Čemu nerozumíš?
What don't you understand? (rozumět takes the dative → čemu)
Čím to otevřu?
What do I open it with? (instrumental of means → čím)
O čem je ten film?
What's the film about? (o takes the locative → čem)
The fixed singular agreement
Here is the quirk that catches everyone. kdo always triggers masculine singular agreement on the verb, no matter how many people the answer turns out to involve, and no matter whether they are male or female. co always triggers neuter singular. You point a verb at kdo or co and it comes out singular, full stop:
Kdo přišel?
Who came? (masc. sg. přišel — even if a whole group arrived)
Kdo chce ještě kávu?
Who wants more coffee? (singular chce, even addressing a roomful of people)
Co se stalo?
What happened? (neuter sg. stalo)
Most strikingly, the past-tense verb after kdo stays masculine even when the person turns out to be a woman. The gender only resurfaces in the answer:
Kdo to byl? — Byla to Petra.
Who was that? — It was Petra. (masc. byl in the question, fem. byla in the answer)
kdo and co as relatives
The same words introduce relative clauses, most often inside the correlative frames ten, kdo (the one who) and to, co (the thing that / what). The demonstrative sets up the antecedent and the pronoun heads the clause:
To, co říkáš, je pravda.
What you're saying is true.
Ten, kdo to udělal, se musí přiznat.
Whoever did it must own up.
Udělám všechno, co můžu.
I'll do everything (that) I can.
Není nikdo, kdo by mu pomohl.
There's nobody who would help him.
Note that the two halves of the frame can carry different cases: in Pomůžu tomu, kdo to potřebuje (I'll help the one who needs it), the demonstrative tomu is dative (governed by pomoct) while kdo is nominative (subject of potřebuje). Each pronoun answers to its own clause.
In everyday speech, Czechs also use co as a catch-all relative meaning that / which, where careful written Czech would use který:
Ten kluk, co bydlí vedle, je z Brna.
The boy who lives next door is from Brno. (colloquial co for který)
This co is fine and extremely common in conversation, but in formal writing you would switch to který (which agrees in gender, number, and case). For the full comparison of the relativizers, see choosing between ten, který, jenž, and co; for který itself, see its declension page.
Common Mistakes
❌ Kdo přišli na schůzku?
Incorrect — kdo always takes a singular verb: přišel.
✅ Kdo přišel na schůzku?
Who came to the meeting?
❌ S kdo jdeš do kina?
Incorrect — the preposition s takes the instrumental: s kým.
✅ S kým jdeš do kina?
Who are you going to the cinema with?
❌ Co se bojíš?
Incorrect — bát se governs the genitive, so what becomes čeho.
✅ Čeho se bojíš?
What are you afraid of?
❌ Koho to dáš?
Incorrect — a recipient is dative; to give to whom is komu, not koho.
✅ Komu to dáš?
Who will you give it to?
❌ O co přemýšlíš?
Incorrect — o takes the locative, so what becomes čem.
✅ O čem přemýšlíš?
What are you thinking about?
Key Takeaways
- kdo and co decline through every case; never leave them in the nominative after a preposition or a case-governing verb.
- kdo: genitive = accusative = koho; instrumental kým, dative komu, locative kom.
- co: soft č- stem in all obliques — čeho, čemu, čem, čím.
- kdo forces masculine singular agreement, co forces neuter singular — regardless of the real number or gender of the answer.
- As relatives they live in the ten, kdo and to, co frames; colloquial co can replace který for that/which.
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Start learning Czech→Related Topics
- který: The Main Relative PronounB1 — který/která/které declined as a hard adjective, the workhorse relative 'which/that/who'.
- jaký vs který: What Kind vs Which OneB1 — Both translate as 'what' or 'which', but jaký asks about quality or kind while který asks you to pick one out of a known set.
- The Indefinite ně- Series: někdo, něco, nějakýA2 — The productive ně- prefix turns question words into indefinites — someone, something, some, somewhere — while the base keeps its own declension.
- Choosing a Relative Word: který, jenž, co, tenB2 — Picking the right relativizer by register and antecedent.
- Question Words and Their CasesA1 — The full set of Czech question words — and the crucial fact that kdo and co decline, so the question word must take the case the verb or preposition demands.