Breakdown of Gdzie jest ta piła, co nią cięliśmy drewno?
być
to be
gdzie
where
ona
she / it (f)
ten
this (m)
Questions & Answers about Gdzie jest ta piła, co nią cięliśmy drewno?
How does the co nią structure work here?
In informal, spoken Polish, you can replace the standard relative pronoun (który) with the unchanging word co plus a regular personal pronoun. Since piła (saw) is feminine, the basic pronoun is ona. The verb requires the Instrumental case for the tool used, so ona becomes nią.
How would this sentence look in a more formal or written style?
You would replace the informal co nią with the appropriate form of który. Because we need the feminine Instrumental form, the sentence would be: Gdzie jest ta piła, którą cięliśmy drewno?
Why exactly is nią in the Instrumental case?
The verb ciąć (to cut) takes the Instrumental case to indicate the tool being used (the 'with what'). Even though English uses the word 'with', Polish just uses the Instrumental case without a preposition. Since nią refers to the saw we cut with, it must be in the Instrumental case.
Why is nią placed right after co instead of after the verb?
Short pronouns like nią naturally want to sit in the second position of a clause, right after the first word (this is a common rhythm rule in Polish). Putting it at the end of the sentence (co cięliśmy drewno nią?) would sound very awkward to a native speaker.