Questions & Answers about Gdzie jest ten młotek, co go wczoraj kupiłem?
Why do we use co go instead of który here?
In colloquial spoken Polish, it is very common to replace the relative pronoun który with the unchanging word co. However, because co doesn't change for case, gender, or number, we often need to add a personal pronoun (like go) to show the grammatical role of the noun in the subclause. Since the speaker bought the hammer (kupiłem go), we use the accusative pronoun go.
How would I say this sentence in formal or written Polish?
You would say Gdzie jest ten młotek, który wczoraj kupiłem?. In the formal version, który takes the accusative case because it is the direct object of kupiłem. For masculine inanimate nouns like młotek, the accusative form is identical to the nominative, so it is just który.
Could I put go at the end of the sentence, like ...co wczoraj kupiłem go?
No, it sounds very unnatural. Unstressed pronouns like go generally cannot be placed at the very end of a sentence if there is another place for them. In these informal co clauses, the pronoun usually tucks in right after co or right before the verb.
Does the form kupiłem imply the speaker is male?