Choosing a Relative Word: který, jenž, co, ten

English has one loose bag of relative words — who, which, that — and, crucially, the option to use no word at all (the book I read). Czech has a small kit of relativizers that are not interchangeable: they differ by register, by what kind of antecedent they attach to, and by whether one is even allowed to appear. This page is the decision hub. Whenever you are about to hang a clause off a noun and you hesitate over který versus jenž versus co, come here. The one-line answer is: use který for almost everything; reach for jenž only in formal writing, co only after a neuter/indefinite antecedent (or in casual speech), and kdo/co only when there is no noun antecedent at all. The rest of the page tells you why, and how the forms behave.

The decision in one flowchart

Ask these questions in order and stop at the first "yes":

1. Is there no noun antecedent — the relative is the whole subject/object ("whoever," "what I need")? → kdo (people) or co (things). Kdo přijde první, vyhraje.
2. Is the antecedent a neuter demonstrative or an indefinite — to, něco, nic, všechno, cokoli? → co. všechno, co vím.
3. Am I writing formal, literary prose and want to vary my style / avoid a third který? → jenž is available (but který is never wrong here).
4. Everything else — any ordinary noun, any register, spoken or written? → který, agreeing with the antecedent.

The workhorse: který

Který (masculine), která (feminine), které (neuter) is a full adjective. It obeys one rule that English never forces you to think about, so internalize it now: který agrees in gender and number with its antecedent, but takes its case from its own clause. The noun it points back to decides what shape family it belongs to; the job it does inside the relative clause decides which ending within that family.

Here kolega is masculine animate singular, so we get the masculine který; and inside the clause it is the subject ("who helped"), so it is nominative. Change the clause-internal role and the case shifts while the gender holds:

In the second, který is the accusative object (kterého, masculine animate); in the third, the preposition s forces the instrumental se kterým. Same antecedent, three different endings — driven entirely by the relative clause, not by the noun outside it. Feminine and neuter antecedents work identically:

The comma is not optional

Before every relativizer in Czech there is a comma. This is not the English "which needs a comma, that doesn't" distinction — Czech marks all subordinate clauses off with commas regardless of whether they are restrictive or non-restrictive. Omitting it is a spelling error, not a style choice.

Both take commas. See subordinate clauses and commas for the wider rule.

The formal twin: jenž

Jenž (masc.), jež (fem./neut.) means precisely the same as který and does precisely the same job. The only difference is register: jenž is written, literary, journalistic. You will read it constantly in books and quality newspapers and essentially never say it aloud. Writers use it to sound polished and, very practically, to avoid a third který stacking up in one paragraph.

Its declension is trickier than který — it draws on the old relative on/jen plus a fixed particle -ž, and after prepositions it takes an n- form: o němž ("about which"), v němž ("in which"), s nímž ("with whom"). Its genitive "whose" forms are jehož (masc./neut.), jejíž (fem.), jejichž (plural), which — unlike English — are the only real single-word "whose" relatives Czech has and are common even in semi-formal writing.

The register swap is the whole point: watch the same sentence move from spoken to printed Czech.

For the full paradigm and the -ž forms, see the literary relative jenž.

After to, všechno, nic: co

When the antecedent is the neuter demonstrative to ("that / the thing") or an indefinite/quantifying word — všechno ("everything"), nic ("nothing"), něco ("something"), cokoli ("anything") — the relativizer is co, not které. This is because you are pointing back to an unspecified "thing," and co ("what") is exactly the word for an unspecified thing. The two members work independently: the head to takes its case from the main clause, while co declines (co, čeho, čemu, čím) according to its role inside the relative clause — to, čemu nerozumím ("the thing I don't understand," dative čemu), to, čeho se bojím ("the thing I'm afraid of," genitive čeho). In the nominative/accusative it simply stays co (to, co říkáš).

There is also a colloquial use of co as a flat, indeclinable stand-in for který after ordinary nouns — ten chlap, co bydlí vedle ("the guy that lives next door"). It is thoroughly idiomatic in relaxed speech and thoroughly out of place in writing.

No noun antecedent: kdo and co

When the relative word does not point back to a noun but is itself the head — "whoever comes," "what I need" — Czech uses kdo for people and co for things. They decline (kdo, koho, komu, kým; co, čeho, čemu, čím) and, characteristically, they trigger singular agreement even with a plural sense, and masculine agreement on any participle.

These correlate naturally with a demonstrative head: ten, kdo… ("the one who…"), to, co… ("the thing that…"). For their full behavior see kdo and co.

The correlative: ten, který / ten, kdo ("the one who")

To say "the one who / the one that," Czech pairs a demonstrative head ten/ta/to in the main clause with a relativizer in the subordinate clause. Both members can decline independently — the ten takes its case from the main clause, the relativizer from its own — which is why this correlative deserves its own treatment.

The full mechanics — including why ten and který/kdo can end up in different cases — are on the ten…který correlative, and the focus-fronting version (to, co mě štve, je…) on clefts and focus.

Why English speakers get this wrong

Two habits transfer badly. First, English drops the relative pronoun — the book I read, the man you saw — and Czech absolutely cannot. There must always be a relativizer, and it must be in the right case.

Second, because English merges who/which/that into one undifferentiated set, learners pick a relativizer by feel and reach for co far too often (it looks like the all-purpose "what"). But after an ordinary noun in neutral or written Czech, the word is který, not co. Save co for the to/všechno/nic frame or for openly casual speech.

Common Mistakes

Key Takeaways

• který / která / které is the default relativizer: correct in every register and after every noun antecedent. It agrees with the antecedent in gender/number and takes its case from its own clause.
• jenž / jež is the literary twin of který — identical meaning, formal-written register only; its genitive "whose" forms jehož / jejíž / jejichž appear even in semi-formal prose.
• co is obligatory after neuter/indefinite antecedents (to, co; všechno, co; nic, co), and is a colloquial replacement for který after ordinary nouns.
• kdo / co are headless relatives ("whoever / what") with no noun antecedent, triggering singular masculine agreement.
• Use the ten…který / ten…kdo correlative for "the one who/that."
• Always put a comma before the relativizer, and never drop it the way English does.