Relative clauses (który, która, które, co) in complex sentences - Polish Grammar

Relative clauses (który, która, które, co) in Polish

What is a relative clause?

A relative clause is a subordinate clause that gives more information about a noun (the antecedent). In English we use who / which / that / what; in Polish we most often use the relative pronouns który / która / które (which/who) and co (what / that which).

Polish example:
- "Mężczyzna, który stoi przy drzwiach, jest lekarzem." — "The man who is standing by the door is a doctor."

When do I use który and when do I use co?

Use który/która/które (and plural forms którzy/które) when you have a definite noun antecedent (a person or thing). Który agrees with that noun in gender, number and takes the case required by its role inside the relative clause.
Example: "Kobieta, którą widziałem, jest znana." — "The woman whom I saw is famous." (kobieta → która → którą because it is feminine and the direct object in the relative clause)

- Use co when the relative clause refers to a whole idea, an indefinite antecedent or words like to / wszystko / nic / coś. Co does not agree in gender/number with a noun and is used like English "what" / "that which".
- Example: "Wiem, co zrobiłeś." — "I know what you did."
- Example: "Wszystko, co kupiłem, było drogie." — "Everything that I bought was expensive."

Important: co should not be used to refer to a specific person in standard Polish. Colloquially some speakers say "ludzie co...", but the standard form is "ludzie, którzy...".

How do I form a relative clause with który? (agreement and case)

Steps: 1) Identify the antecedent (the noun being described) and its gender/number (masculine/feminine/neuter; singular/plural). For people in plural use masculine-personal plural form (którzy) or non-masculine-personal plural (które) for things/animals. 2) Inside the relative clause decide what grammatical role the relative pronoun has: subject, direct object, indirect object (dative), possessor (genitive), object of a preposition (requires locative/instrumental depending on the preposition), etc. 3) Choose the correct case form of który that matches that role inside the relative clause (not the role of the antecedent in the main clause). 4) Place the relative clause right after the antecedent.

Examples by case (Polish → English):
- Nominative (subject inside clause):
- "To jest pies, który szczeka." — "This is the dog that is barking."
- Accusative (direct object inside clause):
- Inanimate: "To jest film, który chcę zobaczyć." — "This is the movie I want to see." (film — który as object; accusative for inanimate = nominative form)
- Animate: "To jest pies, którego widziałem wczoraj." — "That is the dog I saw yesterday." (pies — masculine animate accusative = którego)
- Genitive (possession or after verbs that govern genitive):
- "To jest mężczyzna, którego samochód stoi przed domem." — "This is the man whose car is parked in front of the house."
- Dative (indirect object):
- "Dziecko, któremu dałem prezent, było szczęśliwe." — "The child to whom I gave a present was happy."
- Instrumental / preposition (with prepositions like z, z którym):
- "To jest kolega, z którym pracuję." — "This is the colleague with whom I work." (z + którym; instrumentally marked by preposition z)
- "To jest narzędzie, którym naprawił maszynę." — "This is the tool with which he repaired the machine." (instrumental)
- Locative (with prepositions like w, na):
- "Dom, w którym się wychowałem, jest stary." — "The house in which I grew up is old." (w + którym → locative)

Note on prepositions: In Polish the preposition normally comes before the relative pronoun: e.g. "o którym", "na którym", "z którym", "dla którego".

How do I use co as a relative pronoun?

Co is used when the antecedent is an abstract or indefinite expression (to, wszystko, nic, coś) or when the clause itself acts as a noun phrase.
Basic forms you will see (co declines in oblique cases):
co (nom./acc) — "Wiem, co zrobiłeś." — "I know what you did."
czego (gen.) — "To, czego się boję, to..." — "What I am afraid of is..."
czemu (dat.) — "Nie wiem, czemu on to zrobił." — "I don't know why/for what reason he did it." (can mean "why" or "what for")
czym (instr.) — "Nie wiem, czym się kierował." — "I don't know what he was guided by." (slightly formal)
o czym (loc./with 'o') — "To, o czym rozmawialiśmy, było ważne." — "What we talked about was important."

Examples:
- "To, co powiedziałeś, było ważne." — "What you said was important."
- "Wszystko, co mam, jest twoje." — "Everything that I have is yours."
- "Nic, co mówił, mnie nie przekonało." — "Nothing that he said convinced me."

Remember: do not use co to refer to a specific person. Use który/która/które (or kto in questions) instead.

Defining (restrictive) vs non-defining (non-restrictive) relative clauses

Defining (restrictive) clause: gives essential information that identifies *which* person or thing you mean. It is usually not separated by commas.
Example: "Brat który mieszka w Krakowie przyjedzie jutro." — "The brother who lives in Krakow will arrive tomorrow." (among several brothers, the one from Krakow will come)
Non-defining (non-restrictive) clause: adds extra (non-essential) information about a noun already identified; it is set off with commas.
Example: "Mój brat, który mieszka w Krakowie, przyjedzie jutro." — "My brother, who lives in Krakow, will arrive tomorrow." (I have one brother; extra information about where he lives)

Punctuation tip: Polish punctuation follows the meaning: use commas for parenthetical (non-essential) relative clauses and omit them for essential identifying clauses. Also remember clauses beginning with "to, co" are usually set off by commas: "To, co zrobiłeś, było złe."

Common mistakes to avoid

Using co to refer to people in standard Polish: say "ludzie, którzy..." not "ludzie, co..." (the latter is colloquial).
Forgetting to decline który correctly. The case of który depends on its role inside the relative clause — not on the main clause.
Wrong: "Książka, który leży na stole..." — correct: "Książka, która leży na stole..." (książka is feminine → która)
Confusing masculine animate accusative with inanimate. For animate masculine the accusative equals genitive (np. którego).
Placing the preposition after the clause instead of before the relative pronoun. Correct: "o którym rozmawialiśmy," not "którym rozmawialiśmy o." (Put the preposition before the relative pronoun.)
Omitting the relative pronoun because English allows omission of "that" sometimes. In Polish you usually need który / co.

Quick reference: forms and useful phrases

Common relative constructions:
"to, co" — "that which / what" (e.g. "To, co widziałem, było dziwne.")
"osoba która" / "człowiek który" — "the person who" (use która/který accordingly)
"która z/który z" — "which of" (interrogative: "Który z nich... ?")
"w którym / o którym / z którym / dla którego" — useful prepositional forms
Remember the plural distinction: którzy = plural masculine personal (people); które = plural non-masculine-personal (things, groups)

Glossary (simple definitions)

Antecedent — the noun that the relative clause describes (e.g., "kobieta" in "kobieta która..."
Relative pronoun — word that introduces the relative clause (który / która / które / co)
Restrictive (defining) clause — clause that identifies which one; no commas
Non-restrictive (non-defining) clause — clause that adds extra information; use commas
Case — grammatical form (nominative, genitive, dative, accusative, instrumental, locative) that the relative pronoun must take according to its role inside the relative clause
Agreement — matching gender/number/case between the pronoun and its antecedent

Final tips

When in doubt, find the role of the missing element inside the relative clause (Is it the subject? the object? preceded by a preposition?), then pick the correct case form of który.
Use który/która/które for specific nouns; use co for "what/that which" and for words like "wszystko, nic, coś, to".
Watch punctuation: commas change meaning (defining vs non-defining).

If you would like, I can give a short step-by-step example showing how to build one sentence from English to Polish, choosing the correct form of który and the right case.