Moj sin je danas nemirniji nego jučer.

Moj sin is in the nominative case because it is the subject of the sentence: my son is the one who is more restless.

Moj matches sin in:

• gender: masculine
• number: singular
• case: nominative

That is why you get:

• moj sin = my son
• not moja sin or moje sin

Croatian possessive words must agree with the noun they describe.

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

So:

• sin can mean a son or the son
• moj sin means my son

Croatian usually makes the meaning clear from context instead of using articles.

Je is the 3rd person singular present tense of biti = to be.

So:

• Moj sin je... = My son is...

In this sentence, je links the subject (moj sin) with the description (nemirniji).

The basic adjective is nemiran = restless.

To make the comparative more restless, Croatian forms:

• nemiran → nemirniji

What happens is roughly:

• the adjective stem is nemirn-
• the comparative ending -iji is added

So the a from -an does not stay in the comparative here.

This is why:

• nemiran = restless
• nemirniji = more restless

You should not assume every adjective forms the comparative in exactly the same way, but this pattern is common.

Nego is the normal word for than in many comparisons.

Here it connects the two sides of the comparison:

• danas = today
• jučer = yesterday

So:

• nemirniji nego jučer = more restless than yesterday

Croatian also sometimes uses od in comparisons, especially before nouns or pronouns, but nego is very natural here because the comparison is with a time expression.

Yes. Danas and jučer are adverbs of time:

• danas = today
• jučer = yesterday

They do not decline for case, gender, or number. Their form stays the same.

That is why you simply say:

• danas
• jučer

with no extra endings.

The word order is fairly flexible, but this sentence is very natural.

A key point is that je is a clitic, and Croatian clitics usually go in the second position of the clause.

So:

• Moj sin je danas nemirniji nego jučer. is natural.

You could also rearrange the sentence for emphasis, for example:

• Danas je moj sin nemirniji nego jučer.

That puts more focus on today.

So the word order can change, but je still tends to stay near the beginning, in second position.

Yes, sometimes. Croatian often avoids repeating possession if it is already clear.

A very common alternative is:

• Sin mi je danas nemirniji nego jučer.

Here mi means to me / my in this structure, and the whole phrase means:

• My son is more restless today than yesterday.

Both versions are correct:

• Moj sin... = more explicit, straightforward
• Sin mi je... = very common and natural in everyday speech

A simple approximation is:

ne-mir-ni-yi

More carefully:

• ne
• mir
• ni
• ji

The j in Croatian sounds like English y in yes.

So -ji sounds roughly like yi, not like English j in job.

Also, pronounce all the written vowels clearly:

• ne-mir-ni-ji

Croatian spelling is much more regular than English, so once you know the sounds, the pronunciation is usually predictable.