Learn Croatian: Jeste li vozili auto prekjučer? - Did you drive the car the day before yesterday?

To make a yes/no question in the past tense, you start with the long form of the helper verb biti (to be), followed by the question word li. Here, jeste is the formal or plural "you" form.

Why does the main verb vozili end in -li?

In the past tense, the main verb changes to match the subject. The -li ending is for masculine plural. When using the formal "you" (Vi), we always use the masculine plural ending -li as the default, even if you are politely speaking to just one person.

Why doesn't the word auto change its ending, since it's the direct object?

Auto is indeed in the accusative case because it is the object being driven. However, for masculine nouns that are non-living (inanimate) like a car, the accusative form is exactly the same as the dictionary form.

I recognize jučer (yesterday) inside the word prekjučer. How does this word work?

The prefix prek- comes from preko which means "across" or "over". So prekjučer literally means "over yesterday," giving us the meaning "the day before yesterday".