In short

A relative clause adds information about a noun already named in the main clause: the man who lives here, the book that I read. In Polish the workhorse connector is który (“who / which / that”), and it behaves like an adjective: it copies the gender and number of the noun it points back to, but takes its own case from the job it does inside the relative clause. A second connector, co, steps in after generalising words like to, wszystko, coś, nic and when you comment on a whole idea. Getting który right is one of the clearest markers of moving past beginner Polish.

The rule

A relative clause is a subordinate clause (Lesson 34) that modifies a noun - its antecedent. It is always fenced off by a comma, and the comma comes right before the relative pronoun - or, when a preposition governs it, right before that preposition (Miasto, w którym się urodziłem).

który agrees like an adjective

który declines exactly like a hard adjective (compare dobry / dobra / dobre, Lesson 11). Two independent things decide its ending:

1. Gender and number are inherited from the antecedent - the noun in the main clause it refers to.
2. Case is set by the pronoun’s own role inside the relative clause - subject, direct object, object of a preposition, and so on.

These two choices are separate. Look at the antecedent to pick gender/number; look at what który does in its own clause to pick the case.

• To jest chłopiec, który tu mieszka. = This is the boy who lives here. (antecedent chłopiec -> masc. sg.; który is the subject -> nominative który)
• To jest chłopiec, którego znam. = This is the boy whom I know. (still masc. sg., but który is now the direct object of znam -> accusative/genitive którego)
• To jest chłopiec, któremu pomogłem. = This is the boy whom I helped. (pomóc takes the dative -> któremu)
• To jest dziewczyna, z którą rozmawiam. = This is the girl with whom I am talking. (antecedent dziewczyna -> fem. sg.; preposition z takes the instrumental -> którą)

Prepositions come first

When który is governed by a preposition, the preposition stands in front of który - Polish never strands it at the end the way English can: the house I live in must be dom, w którym mieszkam, never dom, który mieszkam w.

co - the generalising relative

Use co (not który) after neuter generalising antecedents - to, wszystko, coś, nic, coś takiego - and to refer back to a whole preceding clause. co declines like the interrogative co (co, czego, czemu, co, czym, o czym) and again takes its case from its role in the clause.

• To jest wszystko, co wiem. = That is everything (that) I know.
• Zrobił to, czego się bałem. = He did the thing that I was afraid of. (bać się + genitive -> czego)
• Zdała egzamin, co bardzo mnie ucieszyło. = She passed the exam, which made me very glad. (co points at the whole first clause)

Colloquially co is also heard for ordinary nouns (ten pan, co tu mieszka), but in careful/written Polish stick to który for a specific noun and reserve co for the generalising cases above.

Forms & patterns

który takes the full hard-adjective declension. Gender/number come from the antecedent; the case column you actually use is whichever the relative clause demands.

który - singular

Masculine accusative splits by animacy: living referents take the genitive-shaped którego (pies, którego widzę = the dog I see), inanimate ones keep nominative-shaped który (stół, który widzę = the table I see).

który - plural

The plural splits by virility (Lesson 30): the virile (masculine-personal) column vs. the non-virile column for everything else.

co - generalising relative

Examples

Common mistakes

• kobieta, który mieszka obok -> kobieta, która mieszka obok - który must copy the antecedent’s gender (kobieta is feminine -> która).
• chłopiec, który znam -> chłopiec, którego znam - as the object of znam it needs the accusative którego, not nominative.
• dom, który mieszkam w -> dom, w którym mieszkam - the preposition goes before który; Polish never strands it.
• To jest wszystko, które wiem. -> To jest wszystko, co wiem. - after wszystko use generalising co, not który.
• ludzie, które przyszli -> ludzie, którzy przyszli - a masculine-personal (virile) antecedent takes plural którzy.
• pies, który widzę -> pies, którego widzę - an animate masculine object takes the genitive-shaped accusative którego.
• książka który czytam -> książka, którą czytam - a relative clause needs a comma, and the object of czytam is accusative którą.

Tips

• Ask two questions in order: (1) which noun does it point back to? -> that fixes gender and number; (2) what does it do in its own clause? -> that fixes the case.
• Because który is just an adjective, its endings are the ones you already learned in Lesson 11 - który/która/które patterns exactly like dobry/dobra/dobre.
• The comma is non-negotiable and sits immediately before który / co, even when English drops “that”.
• If the antecedent is a vague neuter word (to, wszystko, coś, nic) or a whole clause, reach for co; for a concrete named noun, use który.
• Watch masculine accusative: living -> którego, non-living -> który; and plural virile -> którzy.
• Spot the preposition early - it drags który into whatever case it governs and must lead the clause: z którą, o którym, na których.