Poslije posla ću se osvježiti hladnim tušem.

Because poslije requires the genitive case.

• posao = job / work
• genitive singular of posao = posla

So:

• poslije posla = after work

This is very common in Croatian: certain prepositions force a specific case, and poslije is one of the prepositions that takes the genitive.

Ću is part of the future tense.

Croatian often forms the future with:

• a short form of htjeti (ću, ćeš, će, ćemo, ćete, će)
• plus the infinitive

So:

• ću se osvježiti = I will refresh myself

In this sentence, ću means I will.

Full pattern:

• ja ću
• ti ćeš
• on/ona/ono će
• mi ćemo
• vi ćete
• oni/one/ona će

This is normal Croatian word order with clitics.

Ću is a clitic, a short unstressed word that usually goes near the beginning of the clause, often in the second position. Se is also a clitic.

So Croatian prefers:

• Poslije posla ću se osvježiti.

not:

• Poslije posla osvježiti ću se. ❌

However, if the infinitive comes first, Croatian usually uses the shortened infinitive form:

• Osvježit ću se poslije posla. ✅

So learners should remember:

• ću + infinitive when the infinitive stays later in the sentence
• shortened infinitive + ću when the infinitive comes before ću

Se is the reflexive particle.

The verb here is:

• osvježiti se = to refresh oneself / to freshen up

Without se, the meaning changes or sounds incomplete in this context. The sentence is about the speaker doing something to themselves:

• ću se osvježiti = I will freshen up

English often uses myself only sometimes, but Croatian commonly uses se with verbs like this.

This is about aspect, which is very important in Croatian.

• osvježiti (se) = perfective → a single completed action
• osvježavati (se) = imperfective → ongoing, repeated, habitual action

In this sentence, the speaker means one future action after work:

• I’ll freshen up with a cold shower.

That is why osvježiti se is the natural choice.

If you wanted a repeated or habitual meaning, you might use the imperfective in a different context, for example:

• Poslije posla obično se osvježavam hladnim tušem.
• After work I usually freshen up with a cold shower.

It is the instrumental case.

The phrase hladnim tušem means with a cold shower, where the shower is the means by which the speaker freshens up.

• hladan tuš = a cold shower (nominative)
• hladnim tušem = with a cold shower (instrumental)

Both the adjective and noun change:

• hladan → hladnim
• tuš → tušem

So the sentence literally works like:

• After work, I will refresh myself by means of a cold shower.

Because Croatian can use the instrumental case by itself to express the means or instrument.

In English, we usually need with:

• with a cold shower

In Croatian, that idea can often be expressed just by the instrumental ending:

• hladnim tušem

So no preposition is necessary here.

You may also see s(a) + instrumental in other contexts, but here the bare instrumental is natural and idiomatic.

Yes. Croatian word order is relatively flexible, although not completely free.

The original sentence:

• Poslije posla ću se osvježiti hladnim tušem.

Possible alternatives include:

• Osvježit ću se hladnim tušem poslije posla.
• Hladnim tušem ću se osvježiti poslije posla.

These all mean roughly the same thing, but the focus changes slightly.

• Poslije posla... emphasizes when
• Hladnim tušem... emphasizes how
• Osvježit ću se... emphasizes the action

For learners, the original version is a very natural neutral sentence.

No. You could also use nakon.

• Poslije posla ću se osvježiti hladnim tušem.
• Nakon posla ću se osvježiti hladnim tušem.

Both are correct and natural.

A small note:

• poslije is very common in everyday speech
• nakon can sound a bit more formal or neutral in some contexts

Both take the genitive case, so you still get posla.

In this sentence it means after work in the general everyday sense.

Although posao literally means job/work, the phrase:

• poslije posla

is a very common expression meaning:

• after work
• when I finish work
• after the working day

So it does not usually mean one specific task or assignment unless the context makes that clear.

The original sentence is already natural, but in everyday speech Croatians might also say:

• Poslije posla ću se otuširati hladnom vodom.
  • After work I’ll shower with cold water.

or:

• Poslije posla osvježit ću se hladnim tušem.

Your original sentence is completely good Croatian. It is a nice example because it shows several important patterns at once:

• poslije + genitive
• future tense with ću
• reflexive se
• instrumental for means