Čime se osvježavaš kad je vani tako toplo?

Čime means with what? or by means of what? It is the instrumental singular form of što (what).

So the sentence is literally asking something like:

With what do you refresh/cool yourself when it's so warm outside?

Croatian often uses the instrumental case after the implied idea of using something. That is why you get čime, not što.

Se is the reflexive particle. It shows that the action is directed back to the subject:

• osvježavati = to refresh
• osvježavati se = to refresh oneself / to cool down

In English, we usually do not say refresh yourself every time, but Croatian often uses this reflexive form naturally.

So:

• osvježavaš alone would sound incomplete or would suggest you are refreshing something else
• se osvježavaš means you are refreshing yourself

Osvježavaš is:

• 2nd person singular
• present tense
• from the verb osvježavati se

That means it matches you:

• ja se osvježavam = I refresh myself / I cool down
• ti se osvježavaš = you refresh yourself / you cool down
• on/ona se osvježava = he/she refreshes himself/herself

Because Croatian verb endings already show the subject, the pronoun ti is not necessary.

Kad and kada both mean when.

The difference is mainly style and rhythm:

• kad is shorter and very common in everyday speech
• kada is a bit fuller and can sound slightly more formal or emphatic, depending on context

So both are possible here:

• Čime se osvježavaš kad je vani tako toplo?
• Čime se osvježavaš kada je vani tako toplo?

The first one sounds very natural in conversation.

Vani means outside or outdoors.

In kad je vani tako toplo, it means:

when it is so warm outside

It is an adverb, not a noun, so there is no case ending to worry about here.

You will often hear:

• Vani je hladno. = It is cold outside.
• Djeca su vani. = The children are outside.

Croatian often uses impersonal weather and condition expressions, where English uses it.

So English says:

• It is warm outside.

But Croatian simply says:

• Vani je toplo.

There is no real subject word like English it. The verb is still je (is), but the sentence works without a separate subject.

This is very common with weather and general conditions:

• Hladno je. = It is cold.
• Kasno je. = It is late.
• Toplo je. = It is warm.

Toplo is the neuter singular short form used in impersonal expressions like:

• Hladno je.
• Vruće je.
• Toplo je.

Here, toplo is not describing a masculine or feminine noun. It is part of a general statement about the condition or atmosphere: it is warm.

If you were describing a specific noun, you would match its gender:

• topao dan = a warm day
• topla juha = warm soup
• toplo more = warm sea

But in je tako toplo, the neuter form is the normal one.

Tako here means so, as in so warm.

So:

• tako toplo = so warm

This is a very direct match to English.

Jako usually means very:

• jako toplo = very warm

Both are possible, but they are not exactly the same:

• tako toplo = so warm
• jako toplo = very warm

The original sentence sounds like the speaker is reacting to the situation: when it’s so warm outside...

Not completely. Croatian word order is fairly flexible, but some versions sound more natural than others.

The original:

Čime se osvježavaš kad je vani tako toplo?

is natural and neutral.

You could also hear:

Kad je vani tako toplo, čime se osvježavaš?

This puts more focus on the when it's so warm outside part.

What usually stays together is the reflexive structure:

• se osvježavaš

and the question word often comes early:

• Čime... ?

So the order can shift, but not randomly.

Yes, that is possible, but the nuance is a little different.

• osvježavati se = to refresh oneself, to feel refreshed, to cool off
• rashlađivati se / rashladiti se = to cool oneself down more directly

So:

• Čime se osvježavaš...? sounds natural and everyday
• Čime se rashlađuješ...? sounds more specifically about lowering body temperature

Both can work when talking about hot weather, drinks, shade, swimming, and so on. Osvježavaš se can feel slightly broader and more idiomatic.

Croatian usually drops subject pronouns when the verb ending already makes the subject clear.

Here, osvježavaš already means you refresh yourself / you cool down, so ti is unnecessary.

• Čime se osvježavaš...? = normal
• Čime se ti osvježavaš...? = possible, but adds emphasis

You would include ti if you want contrast or emphasis, for example:

A čime se ti osvježavaš?
= And what do you use to cool down?

So leaving it out is the most natural default.