Questions & Answers about Njegov auto ima običan dizajn.
Why do we use the form običan here instead of obični?
Because we are saying the car has AN ordinary design, not THE ordinary design. Običan is the indefinite adjective form. If we were referring to a specific, previously mentioned design, we would use the definite form obični.
Dizajn is the direct object of the verb ima. Why does it look like it is in the nominative case?
Because dizajn is an inanimate masculine noun (a thing, not a person or animal). For inanimate masculine nouns, the accusative case used for direct objects is exactly the same as the nominative case. The adjective describing it, običan, follows this exact same rule.
If the definite form is obični, why does the indefinite form običan have an extra 'a' in it?
This is called a fleeting 'a'. The core root of the word is 'običn-'. Because Croatian avoids ending words with hard-to-pronounce consonant clusters like '-čn', an 'a' is inserted to make običan. When you add a vowel ending (like the '-i' in obični), that extra 'a' drops out because it is no longer needed for pronunciation.
Does the possessive adjective njegov (his) also have a definite form like descriptive adjectives do?