Learn Polish: Właściciel, który był tu wczoraj, ma duży sklep. - The owner, who was here yesterday, has a big shop.

In English, we use "who" for people and "which" for things. In Polish, kto is generally used as a question word (like "Who is that?"). When you are linking parts of a sentence together (like "The owner, who..."), you must use a form of który, even when referring to a person.

How do I know to use the form który and not która or które?

The relative pronoun który must match the gender and number of the noun it refers back to. Here, it refers to właściciel (owner), which is a masculine singular noun. If the sentence was about a female owner (właścicielka), you would use the feminine form która.

Are the commas around który był tu wczoraj strictly necessary?

Yes! In English, commas before "who" or "which" depend on whether the information is essential or just extra. In Polish, the rule is much simpler: you always put a comma before który. Because this descriptive phrase interrupts the main sentence, you also need a second comma to close it right before the verb ma.

Since ma (has) requires the accusative case, why doesn't duży sklep change its endings?

Duży sklep is indeed in the accusative case here, as the direct object of the verb ma. However, sklep is a masculine inanimate noun (a thing, not a living being). For masculine inanimate nouns, the accusative case looks exactly identical to the nominative (dictionary) form, so the endings don't change.