Kad je vani vruće, osvježavam se hladnim tušem.

Both kad and kada mean when.

• kad is the shorter, very common everyday form
• kada is a bit fuller and can sound slightly more formal or emphatic

In this sentence, Kad je vani vruće... is completely natural Croatian.

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

Croatian often uses this kind of structure for weather or general conditions:

• Vani je hladno. = It is cold outside.
• Vani je toplo. = It is warm outside.
• Vani je vruće. = It is hot outside.

Unlike English, Croatian does not need a dummy subject like it. English says it is hot, but Croatian simply says je vruće.

Here vruće is being used in an impersonal expression, not to describe a specific noun.

Compare:

• Dan je vruć. = The day is hot.
  
  • dan is masculine, so vruć
• Soba je vruća. = The room is hot.
  
  • soba is feminine, so vruća
• Vani je vruće. = It is hot outside.
  
  • impersonal expression, so the neuter-style form vruće

This is very common in Croatian with weather and general states:

• hladno
• toplo
• sunčano
• mračno
• vruće

Vani means outside, outdoors, or outside the house/building, depending on context.

So:

• Vani je vruće. = It is hot outside.
• Djeca su vani. = The children are outside.

It is a very common everyday word.

Se is the reflexive particle. In this sentence, it shows that the action is directed back toward the subject:

• osvježavati = to refresh
• osvježavati se = to refresh oneself / to cool off / to freshen up

So osvježavam se means I refresh myself or more naturally here, I cool off.

Many Croatian verbs are commonly used with se, and the meaning can be different from the non-reflexive version.

Osvježavam is 1st person singular present tense: I refresh / I am refreshing / I cool off.

It comes from the imperfective verb osvježavati se, which is suitable here because the sentence describes a habitual or repeated action:

• Kad je vani vruće, osvježavam se...
  = Whenever it is hot outside, I cool off...

If you used a perfective verb form, it would sound more like a single completed action rather than a general habit.

Because tušem is in the instrumental case, which is often used for the means or instrument by which something is done.

Here, the cold shower is the means by which the person refreshes themself:

• osvježavam se hladnim tušem
  = I refresh myself with a cold shower

Breakdown:

• tuš = shower
• tušem = with a shower / by means of a shower
• hladan = cold
• hladnim = cold, matching instrumental singular masculine

So both words are in instrumental singular:

• hladnim
  • tušem

Because Croatian can use the instrumental case without a preposition to express the means or instrument.

English needs with:

• with a cold shower

Croatian often does not:

• hladnim tušem

This is very normal. Similar examples:

• Pišem olovkom. = I write with a pencil.
• Putujem vlakom. = I travel by train.
• Brišem stol krpom. = I wipe the table with a cloth.

The comma separates the subordinate clause from the main clause:

• Kad je vani vruće, = when it is hot outside
• osvježavam se hladnim tušem. = I cool off with a cold shower

In Croatian, when a sentence begins with a clause introduced by kad, a comma is normally used before the main clause.

Yes, Croatian word order is fairly flexible.

This sentence is natural as:

• Kad je vani vruće, osvježavam se hladnim tušem.

But you could also hear:

• Osvježavam se hladnim tušem kad je vani vruće.
• Kad je vruće vani, osvježavam se hladnim tušem.

The original version is very standard and neutral. Changing the order may slightly change the emphasis, but not the basic meaning.

Here it is best understood as whenever or when(ever) in a habitual sense.

The sentence is not about one single occasion. It describes what the speaker typically does in that situation:

• Kad je vani vruće, osvježavam se hladnim tušem.
• Whenever it’s hot outside, I cool off with a cold shower.

Croatian often uses kad in this broad habitual way.

Yes, grammatically you could, but the meaning changes.

• vruće = hot
• toplo = warm

So:

• Kad je vani vruće... = When it’s hot outside...
• Kad je vani toplo... = When it’s warm outside...

The structure stays the same; only the temperature idea changes.

Yes, it is very natural. It sounds like standard Croatian.

Depending on style and exact meaning, Croatian speakers might also say things like:

• rashladim se hladnim tušem = I cool myself down with a cold shower
• istuširam se hladnom vodom = I take a shower with cold water

But osvježavam se hladnim tušem is perfectly normal and expresses the idea of refreshing/cooling off very well.