Learn Swedish: Bilen är blå och tröjan är vit. - The car is blue and the sweater is white.

In Swedish, the word for "the" is attached to the end of the noun instead of being a separate word. For "en-words" like en bil (a car), you add -en to make bilen (the car). Because en tröja (a sweater) already ends in a vowel, you only need to add -n to make tröjan (the sweater).

Why do we use the basic forms blå and vit here instead of blått and vitt?

Adjectives have to match the gender of the noun they describe. Because both bil and tröja are en-words, the adjectives stay in their basic dictionary form: blå and vit. If we were describing an ett-word, like ett hus (a house), we would have to add a "t" to the adjectives to make them blått and vitt.

If I wanted to ask "Which car?" or "Which sweater?", how would I choose between vilken, vilket, and vilka?

You choose the word for "which" based on the noun's gender and whether it is singular or plural. Since bil and tröja are both singular en-words, you must use vilken. You would ask: Vilken bil är blå? (Which car is blue?) and Vilken tröja är vit? (Which sweater is white?).

Why doesn't end in a consonant like does?