Šesnaestog srpnja želim se sunčati samo ujutro, jer je popodne prevruće.

Because Croatian usually expresses dates with the month in the genitive, and in a sentence like this the day is also commonly in the genitive.

So:

• šesnaesti = sixteenth
• šesnaestog = of the sixteenth / on the sixteenth
• srpanj = July
• srpnja = of July

So Šesnaestog srpnja means on the sixteenth of July.

You will also often see the date written as 16. srpnja.

Srpnja means of July, and it is the genitive form of srpanj.

Croatian month names are different from the international Latin-based ones that English speakers may expect. So:

• srpanj = July
• srpnja = of July

In this sentence, the genitive is needed because of the date expression.

After željeti = to want, Croatian normally uses an infinitive to say what someone wants to do.

So:

• želim = I want
• sunčati se = to sunbathe

Together:

• želim se sunčati = I want to sunbathe

This is similar to English want to + verb.

Because the verb is sunčati se, not just sunčati.

• sunčati se = to sunbathe
• sunčati without se usually means to expose something or someone to the sun

So se is an important part of the verb’s meaning here.

It is also a very common Croatian clitic: a short unstressed word that does not behave like a full independent word in the sentence.

Yes. Ujutro is normally written as one word, and it means in the morning.

So:

• ujutro = in the morning

This is a fixed adverbial expression. English speakers often expect something like u jutro, but for the usual time expression Croatian uses ujutro.

Because samo is limiting ujutro.

• samo ujutro = only in the morning

The position of samo matters. It usually comes right before the word or phrase it is focusing on.

Compare:

• želim se sunčati samo ujutro = I want to sunbathe only in the morning
• samo želim se sunčati = I only want to sunbathe

So in your sentence, samo is specifically restricting the time.

Prevruće means too hot.

It is built from:

• vruće = hot
• pre- = too, excessively

So:

• prevruće = too hot / excessively hot

This pre- prefix is very common in Croatian:

• preskupo = too expensive
• prehladno = too cold
• prekasno = too late

Popodne means afternoon or in the afternoon, depending on context.

In this sentence, it works very naturally as the time period being talked about:

• jer je popodne prevruće = because the afternoon is too hot / because it is too hot in the afternoon

Croatian often uses time words like this without needing extra words that English would usually include.

Yes, both are possible.

• jer = because
• zato što = because

In this sentence, jer is just a short, natural, everyday way to introduce the reason:

• jer je popodne prevruće = because it is too hot in the afternoon

Using zato što would also be correct, but jer sounds more compact here.

Because Croatian does not need a dummy subject like English it in many expressions.

English says:

• it is too hot

Croatian can simply say:

• prevruće je = it is too hot

And in your sentence:

• jer je popodne prevruće

There is no need for a separate word meaning English weather/time it. That is very normal in Croatian.

A rough English-friendly guide would be:

• Šesnaestog ≈ shes-NAES-tog
• srpnja ≈ SRP-nya
• želim ≈ ZHE-lim
• sunčati ≈ SOON-cha-ti
• ujutro ≈ oo-YOO-tro
• prevruće ≈ pre-VROO-che

A few useful sound notes:

• š sounds like sh
• ž sounds like the s in measure
• č sounds like ch in church
• nj sounds like ny in canyon

So srpnja can feel difficult at first because of the consonant cluster, but that is normal for learners.