Ovaj jogurt je kiseliji od jogurta koji obično kupujem.

Ovaj means this.

It is in the masculine singular nominative form because it goes with jogurt, which is a masculine singular noun, and the whole phrase ovaj jogurt is the subject of the sentence.

So:

• ovaj jogurt = this yogurt
• not ovog jogurta, because that would be a different case

Croatian demonstratives change form depending on gender, number, and case.

Je is the 3rd person singular present of biti = to be.

So:

• Ovaj jogurt je kiseliji... = This yogurt is more sour...

Its position is also normal for Croatian because je is a clitic, and clitics usually come near the beginning of the clause, often after the first phrase.

So Ovaj jogurt je... sounds natural.

Kiseliji means more sour, more acidic, or tangier, depending on context.

It is the comparative form of the adjective kiseo = sour.

So:

• kiseo = sour
• kiseliji = more sour

In this sentence it agrees with jogurt, so it is masculine singular nominative, just like ovaj jogurt.

After a comparative adjective like kiseliji = more sour, Croatian commonly uses:

• od + genitive

So:

• kiseliji od jogurta = more sour than the yogurt

This is one of the standard ways to form comparisons in Croatian.

English uses than, but Croatian often uses od plus the genitive case.

Because after od in this comparison, the noun goes into the genitive.

So:

• nominative: jogurt
• genitive: jogurta

That is why the sentence has:

• od jogurta

not:

• od jogurt

It literally means which/that I usually buy.

So the structure is:

• jogurta = of the yogurt
• koji = which/that
• obično = usually
• kupujem = I buy / I am buying

Together:

• od jogurta koji obično kupujem = than the yogurt that I usually buy

Croatian koji can correspond to English that or which. Croatian does not make the same distinction English sometimes does.

Because the form of the relative pronoun depends on its role inside the relative clause, not just on the case of jogurta before it.

In koji obično kupujem, the pronoun is the direct object of kupujem:

• I usually buy which/that

Since jogurt is masculine inanimate singular, its accusative form matches the nominative form here, so koji is correct.

That is why you see:

• koji kupujem

rather than a form used for a different case or for masculine animate nouns.

Croatian often leaves out subject pronouns when they are clear from the verb ending.

So kupujem already means:

• I buy
• or, with obično, I usually buy

You do not need ja unless you want emphasis.

Also, there is no separate word for it because koji already serves as the object inside the relative clause:

• koji kupujem = that I buy

So Croatian does not need an extra object pronoun there.

Because kupujem is the imperfective verb form, and it fits a habitual / repeated action.

With obično = usually, Croatian strongly prefers the imperfective:

• obično kupujem = I usually buy

By contrast, kupim is from the perfective verb kupiti, which focuses more on a completed act. It is not the natural choice for a general habit like this.

So:

• obično kupujem = natural
• obično kupim = not the normal choice here

Not necessarily.

For yogurt, kiseliji often just means:

• more sour
• more tangy
• more acidic

That can be a neutral description of taste. It does not automatically mean the yogurt has gone bad.

Context matters, but in a sentence like this, it most naturally sounds like a taste comparison.

Yes. A very natural alternative is:

• Ovaj jogurt je kiseliji od onog koji obično kupujem.

That means the same thing: This yogurt is more sour than the one I usually buy.

The original sentence is perfectly correct, but this version avoids repeating jogurt/jogurta and can sound a bit smoother if the thing being compared is already clear from context.