Moj sin kaže da je kornjača zanimljivija od zeca i puža.

Because sin (son) is a masculine singular noun, so the possessive my must also be in the masculine singular form: moj.

• moj = my (masculine singular)
• moja = my (feminine singular)
• moje = my (neuter singular)

So:

• moj sin = my son
• moja kornjača = my turtle
• moje dijete = my child

Croatian adjectives and possessives have to agree with the noun in gender, number, and case.

Kaže is the 3rd person singular present tense of kazati / reći in the sense of to say.

Since the subject is moj sin (my son), Croatian uses the he/she/it form:

• ja kažem = I say
• ti kažeš = you say
• on/ona kaže = he/she says

So Moj sin kaže means My son says.

Here da introduces a subordinate clause, similar to English that in My son says that...

So:

• Moj sin kaže = My son says
• da je kornjača zanimljivija... = that the turtle is more interesting...

In everyday English, that is often omitted: My son says the turtle is more interesting...
In Croatian, da is normally kept.

Je is the 3rd person singular present of biti (to be), meaning is.

So:

• da je kornjača zanimljivija = that the turtle is more interesting

Even though English sometimes feels more flexible about structure, Croatian needs the verb je here because this clause has its own predicate:

• kornjača je zanimljivija = the turtle is more interesting

Zanimljivija is the comparative of zanimljiva / zanimljiv (interesting).

Base adjective:

• zanimljiv = interesting

Comparative:

• zanimljiviji = more interesting (masculine)
• zanimljivija = more interesting (feminine)
• zanimljivije = more interesting (neuter)

It ends in -a because it agrees with kornjača, which is a feminine singular noun.

So:

• kornjača je zanimljivija = the turtle is more interesting

In Croatian, one very common way to make comparisons is:

• comparative + od + genitive

So:

• zanimljivija od... = more interesting than...

That is why od is used here.

Examples:

• veći od brata = bigger than the brother
• brži od psa = faster than the dog
• zanimljivija od zeca i puža = more interesting than the rabbit and the snail

Because after od in this kind of comparison, Croatian uses the genitive case.

So the nouns change form:

• zec → zeca
• puž → puža

This is not because they are direct objects; it is because od requires the genitive here.

So:

• od zeca = than the rabbit
• od puža = than the snail

And together:

• od zeca i puža

Usually it is understood as more interesting than the rabbit and the snail, meaning than both of them in the comparison.

In normal usage, a listener will usually understand that the turtle is being compared with both animals mentioned.

If someone wanted to be extra explicit, they could rephrase, but this sentence is perfectly natural as it is.

Croatian does not have articles like English the and a/an.

So:

• sin can mean a son or the son, depending on context
• kornjača can mean a turtle or the turtle
• zec can mean a rabbit or the rabbit

In this sentence, context tells you how to understand it. That is very normal in Croatian.

Yes. Croatian word order is more flexible than English because case endings show the grammatical relationships.

The neutral, natural order here is:

• Moj sin kaže da je kornjača zanimljivija od zeca i puža.

But other orders are possible for emphasis, for example:

• Moj sin kaže da je od zeca i puža kornjača zanimljivija.

This might sound more marked or stylistic, but it is still understandable.

So the original sentence is the most straightforward version, but Croatian can move parts around more easily than English.

Yes, depending on context.

• Sin kaže... = The son / my son / son says... (depending on context)
• On kaže... = He says...
• Moj sin kaže... = My son says...

Croatian often leaves out subject pronouns like on because the verb form already gives information about the person:

• kaže already tells you it is he/she/it says

Here moj sin is included because the speaker wants to name the subject clearly: my son.