Relative pronouns — 'co', 'kiedy' vs 'który' in informal Polish speech

Relative pronouns — 'co', 'kiedy' vs 'który' in informal Polish speech

What this article explains
This guide explains how Polish speakers use three common relative words in everyday (informal) speech: co, kiedy and który. You will learn what each one means, when native speakers use the shorter colloquial forms, when the full form który is required (or recommended), and common learner mistakes to avoid. Polish examples are given with English translations.

What does each word mean?
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[*]co — literally “what / that which.” In standard Polish it introduces clauses like wszystko co, to co, nic co (everything that, that which, nothing that). In informal speech co is also often used as a short relative word instead of który for people and things (colloquial). Co is invariable (it does not change form).
[*]który — “which / who / that.” This is the fully inflecting relative pronoun. It agrees in gender and number with the noun it refers to and takes the case required by its role in the relative clause. Który is the standard choice in written and formal Polish and whenever you need a preposition or a specific case.
[*]kiedy — “when.” Kiedy is an adverb that introduces temporal relative clauses (the time when something happened). In informal speech you will frequently hear kiedy instead of the more formal w którym (in/on which). Kiedy is invariable and cannot appear with prepositions that require case.
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When do I use co? (everyday, short, or standard cases)
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[*]Standard and safe uses:
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[*]After demonstratives or indefinites referring to a thing or idea: to co, tamto co, wszystko co, nic co. Example: 'To, co powiedziałeś, było prawdą.' — (What you said was true.)
[*]With general pronouns: 'Wszystko, co mam, jest twoje.' — (Everything I have is yours.)
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[*]Colloquial use as a substitute for który (very common in spoken Polish):
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[*]'Kolega, co mi pomógł, mieszka niedaleko.' — (The friend who helped me lives nearby.) — spoken, informal.
[*]'Dziewczyna, co stoi przy oknie, jest moją siostrą.' — (The girl standing by the window is my sister.) — informal.
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[*]Important limits: co is invariable and cannot show grammatical case or take prepositions. If the relative clause needs a specific case (or a preposition), co is not appropriate in standard Polish (see "When you must use który").
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When do I use który? (formal / required cases)
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[*]Use który when the antecedent is a specific noun and you want standard / clear grammar (especially in writing).
[*]Use który when the relative pronoun must agree in gender/number or must take a case because of a verb or preposition inside the relative clause. Który behaves like an adjective and declines: który / której / którego / którym / którym (forms depend on gender, number and case).
[*]Use który with prepositions: 'Dom, w którym mieszkam' — (The house in which I live.) or 'Osoba, z którą rozmawiałem' — (The person I spoke with.) You cannot replace those with co in standard language.
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When do I use kiedy? (temporal relative clauses)
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[*]Kiedy introduces time clauses: it means "when" and is used a lot in informal speech as a short alternative to w którym / w którym czasie:
[*]'Pamiętam dzień, kiedy się poznaliśmy.' — (I remember the day when we met.) — informal/casual.
[*]'To był rok, kiedy dużo podróżowałem.' — (It was the year when I traveled a lot.)
[*]But kiedy cannot be used where a preposition must combine with the relative word. For example, you cannot say *'dzień, po kiedy...' — the correct formal phrase is 'dzień, po którym...'. When the temporal expression requires a preposition (po, przed, od, do), use którego/w którym + correct preposition in standard Polish.
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Examples that contrast usage (Polish then English)
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[*]'To, co kupiłem, było drogie.' — (What I bought was expensive.) — co with whole-idea antecedent: correct and standard.
[*]'Wzięliśmy to, które było tańsze.' — (We took the one that was cheaper.) — który points to a particular item among several; co would change meaning.
[*]'Książka, którą kupiłem, leży na stole.' — (The book that I bought is on the table.) — which; correct, standard.
[*]'Książka, co kupiłem, leży na stole.' — (The book I bought is on the table.) — informal; commonly heard in speech but non-standard in writing.
[*]'Mężczyzna, który stoi na rogu, jest policjantem.' — (The man who is standing on the corner is a policeman.) — standard.
[*]'Mężczyzna, co stoi na rogu, jest policjantem.' — (The man standing on the corner is a policeman.) — informal speech.
[*]'Pamiętam dzień, kiedy wyjechałaś.' — (I remember the day when you left.) — informal (kiedy) / natural.
[*]'Pamiętam dzień, w którym wyjechałaś.' — (I remember the day on which you left.) — more formal.
[*]'To jest projekt, o którym (o który) mówiłem.' — (This is the project I was talking about.) — o którym is correct; you cannot say *'o co mówiłem' to mean 'about which' (that would be ungrammatical). In informal speech people might instead rephrase: 'To jest projekt, o którym mówiłem' or 'To jest ten projekt, o którym mówiłem.'
[*]'Koleżanka, której pomogłem, dziękowała mi.' — (The colleague whom I helped thanked me.) — pomoc (dative) requires the correct form której/któremu.
[*]'Koleżanka, co jej pomogłem, dziękowała.' — (colloquial attempt) — many speakers avoid this construction in standard Polish; instead they use który + correct form, or add a clitic pronoun: 'Koleżanka, co jej pomogłem' is awkward; better to say 'Koleżanka, której pomogłem.'
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Common mistakes to avoid
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[*]Using co in formal writing to replace który. In conversations it’s normal, but in essays, emails or exams prefer który (and decline it correctly).
[*]Using kiedy with prepositions. Wrong: *'dzień, po kiedy...' Correct: 'dzień, po którym...'.
[*]Failing to inflect który. Który must agree in gender and number and take the right case: 'książka, którą' (accusative), 'mężczyzna, którego' (genitive), 'dom, w którym' (locative).
[*]Thinking co always equals który. Co is different: it is invariable, often refers to a whole idea, and cannot carry case or preposition meaning. When meaning matters (who did what, who to, who with), use który.
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Short glossary
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[*]Antecedent — the noun or idea that the relative clause describes (e.g., książka in 'książka, którą...').
[*]Relative clause — a subordinate clause that gives more information about a noun or idea (e.g., która kupiłem).
[*]Inflection/decline — changing the form of a word to show case, gender or number (który declines; co and kiedy do not).
[*]Preposition — a word like w, z, o, po that requires a particular case after it (use który + correct case with prepositions).
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Final practical tip
If you are learning Polish: learn który and its common forms so you can use it correctly in writing and in sentences that need case or prepositions. Listen for co and kiedy in conversations — they are natural and frequent — but keep the formal alternatives (który, w którym, po którym) ready for writing and more precise speech.

Good luck — and listen to native speakers: you will quickly pick up when the short spoken forms are used and when the full declined forms are required.