Learn Croatian: Njegov auto koristi dizel umjesto benzina. - His car uses diesel instead of petrol.

It is in the genitive case because of the preposition umjesto (instead of). Prepositions like umjesto, bez (without), and iz (out of) always require the noun that follows them to take the genitive case.

The word auto ends in -o. Why is the possessive pronoun njegov (his) in the masculine form instead of the neuter njegovo?

Even though auto ends in -o, it is a masculine noun. It is actually a shortened form of automobil. Because it is masculine, any adjectives or pronouns describing it, like njegov, must also be in the masculine form.

Why doesn't the direct object dizel change its ending in this sentence?

Dizel is in the accusative case because it is the direct object of the verb koristi. For masculine inanimate nouns (which include things, materials, and concepts), the accusative form is exactly the same as the nominative dictionary form.

Is the verb koristi perfective or imperfective in this context?

Here it acts as an imperfective verb, describing a general, ongoing fact about the car. The verb koristiti is actually biaspectual, meaning the exact same word can be used for both perfective (completed action) and imperfective (ongoing action) meanings depending on the context.