Learn Swedish: Vilken tröja är vit? - Which sweater is white?

Vilken means "which" and it changes to match the gender of the noun. Since tröja (sweater) is an en-word (en tröja), we use vilken. If we were asking about an ett-word, we would use vilket.

How would this sentence change if I wanted to ask about multiple sweaters?

The word for "which" changes to vilka for all plural nouns. You would also need to make the noun and adjective plural, so "Which sweaters are white?" becomes Vilka tröjor är vita?.

Why is the adjective spelled vit and not vitt or vita?

Just like the question word vilken, adjectives have to match the noun they describe! Because en tröja is a singular en-word, the adjective stays in its basic form, vit.

Does tröja only mean "sweater"?

Tröja is a versatile word. While it usually means "sweater" or "jumper", it can also refer to a general top or a t-shirt (t-tröja). For a formal, button-up dress shirt, you would use the word skjorta instead.