Questions & Answers about Moja synowa, z którą mieszkam, jest bardzo miła.
Why is the relative pronoun in the form którą instead of która?
The pronoun refers back to synowa, which is feminine. However, it comes right after the preposition z (meaning "with"). When z means "with", it requires the Instrumental case. The feminine Instrumental form of która is którą.
Are the commas around z którą mieszkam strictly necessary?
Yes! Unlike English, which sometimes lets you drop commas around "who" or "which" clauses, Polish grammar strictly requires you to separate relative clauses from the rest of the sentence with commas.
Is the word synowa (daughter-in-law) based on the word syn (son)?
Yes, exactly. Polish family vocabulary is often built logically from root words. A synowa is the wife of your syn (son).
Why do we use the third-person verb jest right after the first-person verb mieszkam?
Because they belong to two different parts of the sentence. The main idea is Moja synowa [...] jest bardzo miła (My daughter-in-law is very nice). The phrase z którą mieszkam is just an inserted clause, so the verb jest still matches the main subject, synowa.