Kolač s lješnjakom skuplji je od kolača s orasima.

Because the two nouns have different grammatical roles.

• kolač is the subject of the sentence, so it is in the nominative singular.
• kolača comes after od in a comparison, and od requires the genitive case here.

So:

• kolač = nominative
• od kolača = genitive after od

This is very common in Croatian comparatives:

• Ivan je viši od Marka.
• Ova knjiga je bolja od one knjige.

Because s meaning with requires the instrumental case, and the noun endings change depending on number.

• lješnjak is singular, so instrumental singular is lješnjakom
• orasi is plural, so instrumental plural is orasima

So:

• s lješnjakom = with a hazelnut / with hazelnut
• s orasima = with walnuts

The key grammar point is:

• s + instrumental

Examples:

• s prijateljem = with a friend
• s prijateljima = with friends

This can feel strange to an English speaker, because English often does not care much about singular vs. plural in ingredient labels in the same way.

In Croatian, ingredient expressions are not always perfectly parallel. Both singular and plural can occur depending on how the speaker imagines the ingredient:

• s lješnjakom can mean with hazelnut as an ingredient/flavour
• s orasima often means with walnuts, thought of as multiple pieces

So this is not a strict logical contrast like one hazelnut versus many walnuts. It is more about natural usage.

In real Croatian, you may also hear other patterns depending on context, recipe style, or personal preference.

Skuplji is the comparative form of the adjective skup (expensive).

• skup = expensive
• skuplji = more expensive

So the sentence uses a comparison:

• skuplji je od ... = is more expensive than ...

This is a regular Croatian pattern:

• brz → brži = faster
• lijep → ljepši = more beautiful
• skup → skuplji = more expensive

Notice that the comparative adjective must agree with the noun it describes. Here it matches kolač, which is masculine singular.

Croatian word order is flexible, and this order is very natural.

• Kolač s lješnjakom skuplji je od kolača s orasima.
• Kolač s lješnjakom je skuplji od kolača s orasima.

Both are possible.

The version with skuplji je is a very normal Croatian word order. Short unstressed words like je often appear in the second position area of the clause, following the first stressed unit or phrase.

So this is not a different meaning, just a natural Croatian placement.

Yes.

You can say:

• Kolač s lješnjakom je skuplji od kolača s orasima.

That is also correct and natural.

The version you were given:

• Kolač s lješnjakom skuplji je od kolača s orasima.

sounds slightly stylistic or neutral-literary, but still very normal. Croatian often allows more than one acceptable word order.

In this kind of comparison, Croatian usually uses:

• comparative + od + genitive

So od is the normal word for than in noun comparisons.

Examples:

• veći od kuće = bigger than the house
• bolji od mene = better than me
• skuplji od kolača s orasima = more expensive than the cake with walnuts

English uses a special comparison word, than. Croatian often uses od instead.

Usually od is the best choice here, because it is followed by a noun phrase:

• od kolača s orasima

Nego is more typical when what follows is more clause-like, or in some fixed comparative structures.

So in this sentence:

• skuplji od kolača s orasima = the normal choice

A learner should first remember:

• comparative + od + genitive for straightforward noun comparisons

Kolača is in the genitive singular.

It is genitive because it follows od in a comparison:

• skuplji od kolača

This is an important pattern:

• od + genitive

Compare:

• od brata = than/from the brother
• od grada = than/from the city
• od kolača = than/from the cake

In this sentence, the meaning is clearly comparative: more expensive than the cake with walnuts.

Here s means with.

So:

• kolač s lješnjakom = cake with hazelnut
• kolač s orasima = cake with walnuts

Be careful: s(a) can also mean with in the sense of accompanying someone:

• s prijateljem = with a friend

The grammar is the same: s + instrumental.

Not always.

Kolač is a general word for a cake, pastry, or sweet baked item. In Croatian, it can cover a wider range than English cake.

Depending on context, Croatian may also use:

• torta = a layer cake / celebration cake
• kolač = cake, pastry, sweet baked item

So in this sentence, kolač is perfectly fine, but in another context English cake might be torta instead.

Because Croatian has no articles.

English distinguishes:

• a cake
• the cake

Croatian usually does not use a separate word for that. The meaning comes from context.

So:

• kolač can mean a cake or the cake
• kolača can mean of a cake or of the cake, depending on context

That is completely normal in Croatian.

A few sounds may be unfamiliar to an English speaker:

• lj is pronounced as a single soft sound, similar to the lli in some pronunciations of million
• je here sounds roughly like ye
• š sounds like sh
• nj is a soft sound similar to ny in canyon

So lješnjakom is approximately:

• lyesh-nya-kom

That is only an approximation, but it can help at first.

Grammatically, yes, it is singular:

• Kolač ... skuplji je od kolača ...

But in real use, Croatian singular can also describe a type of product:

• A hazelnut cake is more expensive than a walnut cake
• The cake with hazelnut is more expensive than the one with walnuts

So the exact interpretation depends on context. The sentence can refer to:

1. two specific cakes, or
2. two kinds of cake

Both are possible.

Yes, very often, if the meaning is clear.

For example:

• Kolač s lješnjakom skuplji je od onog s orasima.

This means:

• The cake with hazelnut is more expensive than the one with walnuts.

Croatian often avoids repeating a noun when context makes it obvious.

The full version with kolača is still completely correct and clear.