Moj brat jede hrenovku sa senfom, ali sestra kaže da je majoneza bolja od senfa.

Because hrenovku is the direct object of jede. In Croatian, the direct object usually goes in the accusative case.

• nominative: hrenovka
• accusative: hrenovku

So Moj brat jede hrenovku means My brother is eating a hot dog / frankfurter.

This is a very common pattern:

• Imam knjigu = I have a book
• Vidim sestru = I see my sister

Because sa/s meaning with takes the instrumental case here.

• base form: senf
• instrumental singular: senfom

So:

• sa senfom = with mustard

This is a standard case pattern:

• s prijateljem = with a friend
• sa sirom = with cheese

Croatian uses sa instead of s before certain sounds, especially when the next word starts with s, š, z, or ž, or when s would be awkward to pronounce.

So:

• sa senfom sounds natural
• s senfom is avoided

Compare:

• s bratom = with my brother
• sa sestrom = with my sister

Because Croatian often leaves out a possessive like moj / moja when it is already clear from context.

So in this sentence, sestra is naturally understood as my sister because the speaker already mentioned my brother and is talking about family members.

You could say moja sestra, but it is not necessary here.

Here da introduces a subordinate clause and means that.

So:

• kaže da je... = says that ... is ...

This is extremely common in Croatian:

• Mislim da je dobro = I think that it is good
• Znam da dolazi = I know that he/she is coming

In everyday English, that is often omitted, but in Croatian da is normally used.

Because je is a clitic in Croatian. Clitics are short unstressed words that usually appear very early in their clause, often in the second position.

In the clause:

• da je majoneza bolja od senfa

da comes first, and the clitic je comes right after it.

This word order is very normal in Croatian, even if it feels unusual to an English speaker.

Because bolja is the comparative form of dobra / dobar / dobro meaning better, and it must agree with majoneza, which is feminine singular.

• masculine: bolji
• feminine: bolja
• neuter: bolje

Since majoneza is feminine:

• majoneza je bolja = mayonnaise is better

After a comparative like bolji / bolja / bolje meaning better, Croatian commonly uses od + genitive to mean than.

So:

• bolja od senfa = better than mustard

Here:

• base form: senf
• genitive singular: senfa

More examples:

• veći od mene = bigger than me
• brži od auta = faster than the car

Because it has two different grammatical roles, so it takes two different cases.

1. sa senfom

   • after sa
   • instrumental case

2. od senfa

   • after od
   • genitive case

This is a good example of how Croatian changes noun endings depending on function.

Croatian does not have articles like a and the.

So Croatian simply says:

• brat
• sestra
• majoneza
• senf

Whether something is a brother, the brother, mustard, or the mustard is understood from context.

That is completely normal in Croatian.

Usually hrenovka means the sausage itself, like a frankfurter or hot-dog sausage.

So literally, the sentence is talking about eating a hrenovka with mustard. In some contexts, English speakers might naturally think of a hot dog, but the Croatian word itself usually refers to the sausage more directly than the whole bread-and-sausage item.

Yes, jede is present tense of jesti = to eat.

In Croatian, the present tense can often correspond to both:

• eats
• is eating

So Moj brat jede hrenovku can mean:

• My brother eats a hot dog
• My brother is eating a hot dog

The exact meaning depends on context.

A few useful pronunciation points:

• ž sounds like the s in measure
• j usually sounds like English y
• e is a clear eh sound

So:

• kaže sounds roughly like KAH-zheh
• majoneza sounds roughly like mah-yo-NEH-zah

Also:

• senf is close to senf
• brat is like braht
• sestra is SEH-stra

These are only rough English approximations, but they help at the start.