Questions & Answers about Gdzie jest dziewczyna, którą zaprosiłeś do kina?
Why do we use którą here instead of the dictionary form która?
Którą is the accusative (direct object) form of the feminine pronoun która. We use the feminine form because it refers back to dziewczyna (the girl). Even though dziewczyna is the subject in the first half of the sentence, in the second half she is the direct object of the verb zaprosić (to invite). Because of that, the connecting pronoun must take the accusative case.
Can I skip the word którą like we often skip 'that' or 'who' in English?
No, you can never skip the relative pronoun in Polish! While in English you can say 'the girl you invited', in Polish you must always include którą. Also, notice the comma before it: Polish grammar strictly requires a comma to separate the main sentence from the relative clause.
How do we know who did the inviting just from the word zaprosiłeś?
Why does the word kino change to kina at the end of the sentence?