co

The relative pronoun co serves two primary roles in Polish:

1. Standard Usage
You must use co (and its declined forms) when the antecedent is a demonstrative pronoun like to (that), or an inanimate indefinite/negative pronoun like wszystko (everything), coś (something), cokolwiek (anything/whatever), and nic (nothing). It is also commonly used after taki (such) and tyle (so much/many), though jaki (for taki) and ile (for tyle) are the standard, precise pairs.

• Preposition Rule: In standard relative clauses, prepositions must precede the declined form of co (e.g., o czym, z czym, do czego). Unlike English, Polish never strands prepositions at the end of a clause (e.g., 'the thing I was thinking about' becomes to, o czym myślałem).
• Comma Rule: Just like with który, a comma marks the start of the relative clause, before the whole introductory phrase: to, o czym mówisz.
• Sentence Reference: Use co when referring back to an entire previous clause (e.g., 'He forgot his keys, which annoyed me').

In this role, co declines like the question word co (co, czego, czemu, co, czym, czym).

2. Colloquial Usage
In informal spoken Polish, co often replaces który to refer to specific people or objects. In this case, co usually remains invariant (it doesn't change its form).

• In the nominative case, co is used alone without a resumptive pronoun (e.g., chłopak, co tam stoi).
• If the grammar requires a case other than the nominative, speakers often add a resumptive pronoun (like go, ją, ich) to show the case. Example: Ten chłopak, co go znasz (That boy that you know [him]).
• Prepositions with Resumptive Pronouns: When a preposition is required colloquially, it attaches to the resumptive pronoun, not to co (e.g., ten chłopak, co z nim rozmawiałem, not z co nim).
• Omission of Resumptive Pronouns: The resumptive pronoun is frequently dropped if the referent is an inanimate object (e.g., książka, co czytam) especially when it is a direct object: książka, co czytam corresponds to formal książka, którą czytam. Do not rely on matching the antecedent’s case to predict omission in every construction.
• Restrictive vs. Non-Restrictive Clauses: Colloquial co is primarily used in restrictive relative clauses (clauses that define the noun). Using it in non-restrictive/appositive clauses (e.g., 'Mój brat, co mieszka w Warszawie...') is prescriptively frowned upon, and który is strongly preferred, though native speakers sometimes still use co here in very casual speech.

People Versus Things: In standard clauses after personal pronouns, use kto and its case forms: ktoś, kto wie (someone who knows), ten, kogo szukasz (the person you are looking for). Use co after thing-reference forms such as coś and wszystko. This standard distinction is separate from colloquial invariant co after a noun.

When co refers to a whole preceding fact, its own clause still determines the case and preposition: Wygrała konkurs, z czego bardzo się cieszymy (She won the competition, which we are very pleased about). Here z czego refers to her winning, not to a particular noun.