Questions & Answers about To mój szwagier, z którym pracuję.
Why do we use którym instead of the dictionary form który?
The relative pronoun has to match the gender and number of the noun it refers back to (szwagier is masculine singular). However, its case is decided by what's happening inside its own clause. Here, the preposition z (meaning "together with") forces the Instrumental case. The Instrumental masculine singular form of który is którym.
Can I put the preposition at the end like in English and say którym pracuję z?
No, Polish never leaves a preposition dangling at the end of a sentence like English often does ("who I work with"). The preposition z must sit directly in front of the pronoun it modifies, which is why the sentence structure strictly translates to "with whom I work".
Why is there a comma before z którym?
In Polish, you must always use a comma to separate the main sentence from a relative clause (a descriptive part of the sentence starting with który, co, kto, etc.). While English punctuation might skip a comma here, Polish rules strictly require it.
Is it okay if I add jest and say To jest mój szwagier?
Yes, that is perfectly fine and grammatically correct. However, when using the pronoun to to introduce someone or point something out ("This is..."), Polish speakers frequently drop the verb jest because it is faster and sounds very natural in casual speech.