The casual alternative to "który"
In formal Polish, to say "the hammer that I bought," you would use a declined form of the relative pronoun który (which/that). In casual, spoken Polish, there is a highly common shortcut: using the invariant word co at the start of the clause, followed by an ordinary personal pronoun.
Because co never changes its form, the added pronoun (like go for masculine or ją for feminine) acts as a placeholder that shows the grammatical case. Think of it literally as "the hammer that I bought it" or "the drill that I got it."
| Where is that hammer that I bought yesterday? | Gdzie jest ten młotek, co go wczoraj kupiłem? |
| This is the drill that I got from my brother. | To jest ta wiertarka, co ją dostałem od brata. |
Handling plurals
This pattern works smoothly across all genders and numbers. When talking about plural tools like kombinerki (pliers), use the plural direct object pronoun je.
| Give me those pliers that I saw on the desk. | Daj mi te kombinerki, co je widziałem na biurku. |
Using other grammatical cases
The flexibility of the co + pronoun construction means you can use any case your verb or preposition requires, just by swapping out the pronoun.
If you are cutting wood with a piła (saw), you use the Instrumental case. Since piła is feminine, you pair co with the feminine Instrumental pronoun nią (with it/her).
| Where is that saw that we cut the wood with? | Gdzie jest ta piła, co nią cięliśmy drewno? |
Similarly, if you talk about a guy (facet) you lent string to, you need the Dative case for the recipient. Pair co with the masculine Dative pronoun mu (to him).
| The guy you lent the string to is drilling a hole. | Facet, co mu pożyczyłeś sznurek, wierci dziurę. |