Za večeru ćemo napraviti hrenovke; ja volim hrenovku s kečapom, a ona s majonezom.

Because za often takes the accusative case when it means for.

The base form is večera = dinner / supper.
Its accusative singular form is večeru.

So za večeru means for dinner.

Ćemo is the 1st person plural future auxiliary, meaning we will.

Croatian future tense is often formed with: auxiliary of htjeti + infinitive

So:

ćemo napraviti = we will make

As for word order, Croatian clitic words like ćemo usually go in the second position of the sentence or clause. Since the sentence begins with Za večeru, the clitic comes right after that phrase:

Za večeru ćemo napraviti hrenovke.

That placement is normal Croatian word order.

Napraviti is a perfective verb, while praviti is imperfective.

In this sentence, the speaker is talking about a single completed action in the future: making dinner. Croatian usually prefers a perfective verb for that kind of meaning.

So:

napraviti = to make, to prepare, as a completed action
praviti = to be making / to make in a more ongoing or habitual sense

That is why napraviti sounds natural here.

Hrenovka usually means a frankfurter / hot dog sausage / wiener-type sausage.

Depending on context, English might translate it as:

• sausage
• hot dog
• frankfurter

In this sentence, it refers to the food item itself, not necessarily the whole bun-and-sausage combination.

Also, hrenovka is a feminine noun.

Because they are different number/case forms of the same noun.

The dictionary form is hrenovka.

Here:

napraviti hrenovke
This means make hot dogs / sausages, plural.
After napraviti, the direct object is in the accusative plural, which here is hrenovke.

volim hrenovku
This means I like a hot dog / sausage, singular.
After volim, the direct object is in the accusative singular, which is hrenovku.

So the difference is simply:

• hrenovke = plural
• hrenovku = singular

Because the preposition s meaning with takes the instrumental case.

So:

• kečap becomes kečapom
• majoneza becomes majonezom

That is why you get:

s kečapom = with ketchup
s majonezom = with mayonnaise

This is a very common pattern in Croatian: s + instrumental

The verb is understood and left out because it would repeat information that is already clear.

The full version would be something like:

Ja volim hrenovku s kečapom, a ona voli hrenovku s majonezom.

But Croatian often omits repeated words when the meaning is obvious. So:

a ona s majonezom

means:

and she likes hers with mayonnaise
or more literally, and she [likes a hot dog] with mayonnaise

This kind of ellipsis is very natural.

Here a links two contrasting pieces of information.

In this sentence, it is similar to:

• and
• while
• sometimes but

So:

ja volim hrenovku s kečapom, a ona s majonezom

means something like:

I like a hot dog with ketchup, while she likes it with mayonnaise.

It is not a strong contradiction; it is more a contrast between two preferences.

Ona is useful and basically needed here, because the subject changes and the speaker wants to make the contrast clear: I ... she ...

Ja is not strictly necessary grammatically, because volim already means I like. Croatian often drops subject pronouns.

So you could also say:

Volim hrenovku s kečapom, a ona s majonezom.

But keeping ja adds emphasis and makes the contrast stronger:

ja ... a ona ...

That sounds very natural in this sentence.