koji vs što (relative 'which/that')

English uses which, that and who somewhat interchangeably to start a relative clause. Croatian has two main relative words and they are not interchangeable: koji and što. The deciding question is what the relative word refers back to. If it points to a specific noun (a particular town, a particular woman), you want koji — and crucially, koji then changes its ending to fit its job inside the relative clause. If it points to a whole idea or clause (something that just happened), or sits with words like sve / nešto / ništa, you want the invariant što. For the full mechanics of building relative clauses see relative clauses; for the complete koji declension see the relative koji.

The core split

The first job needs an agreeing, case-marked word, because the antecedent is a noun with gender, number, and a grammatical role; only koji can carry that. The second job has no single noun to agree with — there's nothing to take a gender or a case from — so the unchanging što does the work.

koji — for a specific noun, and whenever a case or preposition is needed

Use koji when the relative clause modifies a particular noun. The key skill: koji takes its gender and number from the antecedent, but its case from its role inside the relative clause. In žena koju volim („the woman whom I love"), koju is feminine singular (from žena) but accusative (because it is the object of volim inside the clause).

This is exactly why koji is obligatory when a preposition is involved: the preposition has to attach to a word that can take its case, and only koji inflects. You cannot say *grad u što živim — što has no locative form for u to govern. English happily strands the preposition („the town I live in"); Croatian pulls the preposition to the front and puts koji in the case it demands: grad u kojem živim.

što — for a whole-clause antecedent

Use što when the relative clause comments on the entire preceding clause rather than on a single noun. English does this with „which": „He was late, which annoyed me" — the which refers to the whole fact of his being late, not to any noun. Croatian uses što for this, invariant.

Note the typical pattern: the što-clause is set off by a comma and the verb's participle is neuter (naljutilo, obradovalo), because a whole clause counts as a neuter „it".

što — with sve, nešto, ništa, ono

After the quant/indefinite words sve („everything / all"), nešto („something"), ništa („nothing") and the demonstrative ono („that which"), the relative word is što, not koji. These antecedents are not concrete gendered nouns, so the invariant što fits.

Colloquial što as a general relative (nonstandard)

In casual speech, što is often pressed into service as an all-purpose relative even for ordinary noun antecedents, usually paired with a resumptive pronoun that carries the case što cannot. Čovjek što sam ga vidio („the man that I saw him") uses što plus the resumptive clitic ga („him") to do the accusative job. This is widespread in spoken Croatian but nonstandard — in writing and careful speech use koji.

Common Mistakes

Key Takeaways

• koji for a specific noun antecedent: it agrees in gender/number with the noun but takes its case from its role inside the relative clause (žena koju volim, grad u kojem živim).
• koji is obligatory whenever a preposition is involved — Croatian fronts the preposition and inflects koji to its case (s kojim, o kojoj); što cannot do this.
• što (invariant) for a whole-clause antecedent („…, which…"), with a neuter participle (Zakasnio je, što me naljutilo).
• što also after sve, nešto, ništa, ono (sve što imam, nešto što ti moram reći).
• Colloquial što as a catch-all relative + resumptive pronoun (čovjek što sam ga vidio) is common in speech but nonstandard — write koji.