Moj unuk kaže da mu je hrvatski najzanimljiviji predmet u školi.

Because moj has to agree with unuk.

• unuk = grandson
• It is masculine singular nominative
• So the possessive word must also be masculine singular nominative: moj

Compare:

• moj unuk = my grandson
• moja unuka = my granddaughter
• moje dijete = my child

So moj is chosen because of the gender, number, and case of unuk.

Kaže means says, and da introduces the statement being said.

So:

• Moj unuk kaže = My grandson says
• da... = that...

Very literally:

• Moj unuk kaže da... = My grandson says that...

In everyday English, that is often omitted, but in Croatian da is normally used in this kind of sentence.

Because in Croatian, with verbs like reći, kazati, misliti, znati, vidjeti, a following da-clause usually does not take a comma.

So this is standard:

• Moj unuk kaže da mu je hrvatski najzanimljiviji predmet u školi.

Not:

• Moj unuk kaže, da...

This is different from some English punctuation habits, where learners may expect a pause or comma more often.

Mu means to him. It is the dative short form of on / njemu.

So the sentence is literally something like:

• My grandson says that to him Croatian is the most interesting subject in school.

In natural English, we usually just say:

• My grandson says Croatian is the most interesting subject at school.

But Croatian often uses the dative like this to show whose opinion or experience it is.

Yes, you can, but the nuance changes a little.

• Moj unuk kaže da mu je hrvatski najzanimljiviji predmet u školi. = My grandson says that, for him, Croatian is the most interesting subject at school.

• Moj unuk kaže da je hrvatski najzanimljiviji predmet u školi. = My grandson says Croatian is the most interesting subject at school.

The version with mu sounds a bit more personal and clearly marks it as his view.

Because mu and je are clitics: short unstressed words that usually stand very early in the clause.

After da, Croatian naturally puts them near the beginning:

• da mu je hrvatski...

This sounds normal in Croatian, even though English would place words differently.

So you should think of mu je as a fixed little clitic group that wants to come early, rather than translating word by word from English.

Because in Croatian, names of languages and school subjects are normally written with a lowercase letter.

So:

• hrvatski = Croatian
• engleski = English
• matematika = mathematics

It would only be capitalized if it came at the beginning of a sentence.

Formally, it is an adjective, but here it is being used on its own as a shortened form of hrvatski jezik.

So:

• hrvatski jezik = the Croatian language
• hrvatski = Croatian

In school context, Croatian speakers often shorten the subject name this way, just like English speakers say math instead of mathematics.

It is the superlative form of zanimljiv = interesting.

The pattern is:

• zanimljiv = interesting
• zanimljiviji = more interesting
• najzanimljiviji = most interesting

So naj- is added to the comparative form to make the superlative.

Here it agrees with predmet:

• predmet is masculine singular
• so najzanimljiviji is also masculine singular

Here predmet means school subject.

So:

• najzanimljiviji predmet u školi = the most interesting subject in school / at school

Be careful, because predmet can also mean object in other contexts. Croatian uses the same word for both ideas, and the context tells you which meaning is intended.

Because u here means in / at, describing location, so it takes the locative case.

• base form: škola = school
• locative singular: školi
• therefore: u školi = in school / at school

This is a very common pattern:

• u kući = in the house
• u gradu = in the city
• u školi = at school

Croatian has no articles like English a and the.

So najzanimljiviji predmet can mean:

• the most interesting subject

without any separate word for the.

Croatian usually shows definiteness from context rather than from an article.

Croatian word order is fairly flexible, but it is not completely free.

This sentence is natural as:

• Moj unuk kaže da mu je hrvatski najzanimljiviji predmet u školi.

You can move some parts for emphasis, but the clitics mu and je still need to stay in their usual early position.

For example, a more emphatic version might be:

• Moj unuk kaže da je hrvatski najzanimljiviji predmet u školi.
• Moj unuk kaže da je njemu hrvatski najzanimljiviji predmet u školi.

So the main idea is:

• content words can move around more than in English
• clitics like mu and je are much less movable