Learn Croatian: Imamo udoban automobil. - We have a comfortable car.

They actually are in the accusative (direct object) case! The rule for masculine nouns representing inanimate objects (things like cars, not living beings) is that their accusative form looks exactly the same as their dictionary (nominative) form. The adjective udoban follows the same rule to match the noun.

If we were talking about a living being instead of a car, how would the adjective change?

For masculine animate nouns (people and animals), the accusative endings change. The noun would usually get an -a at the end, and the adjective would get -og or -oga. Since a car is inanimate, we stick to the basic dictionary form: udoban automobil.

Where is the word for "we" in this sentence?

In Croatian, you usually drop the pronoun (like mi for "we") because the verb ending gives it away. The -mo at the end of imamo tells you exactly who is having the car: we are!

Why is there no word for "a" before udoban automobil?

Croatian doesn't use articles like "a", "an", or "the". A phrase like udoban automobil can translate to "a comfortable car" or "the comfortable car". You just rely on the context to know which one makes sense in English.