Kad je vani hladno, puhnem u dlanove.

Yes. Kad and kada both mean when.

• kad is more common in everyday speech
• kada can sound a bit more formal, careful, or emphatic

So Kad je vani hladno and Kada je vani hladno both work.

Because kad means when / whenever, while ako means if.

In this sentence, the idea is a normal, repeated situation:

• Kad je vani hladno, puhnem u dlanove. = Whenever it’s cold outside, I blow into my palms.

If you said:

• Ako je vani hladno, puhnem u dlanove.

that would sound more conditional: If it’s cold outside, then I blow into my palms.

Both are possible in some contexts, but kad is very natural when talking about something that regularly happens.

Croatian does not need a dummy subject like English it in weather or general-environment expressions.

English says:

• It is cold outside.

Croatian simply says:

• Vani je hladno.
• literally something like Outside is cold

So there is no separate word corresponding to English it here.

Here hladno is the form Croatian uses in impersonal expressions such as:

• hladno je = it is cold
• toplo je = it is warm
• mračno je = it is dark

You can think of it as the neuter singular form used in a general, non-personal way.

It is not agreeing with a noun here, because there is no noun like day, weather, or air in the sentence. The expression just means it is cold.

Vani means outside or outdoors.

Croatian word order is flexible, so you can also hear:

• Vani je hladno.
• Hladno je vani.
• Kad je vani hladno, ...
• Kad je hladno vani, ...

These all mean basically the same thing, though the emphasis may shift slightly.

Vani is a very common everyday word. Depending on region, you may also hear words like napolju, but vani is standard and very common.

Because Croatian usually drops subject pronouns when the verb form already shows who is doing the action.

Puhnem already means I blow because the ending tells you it is first person singular.

So:

• puhnem = I blow

You would add ja only for emphasis or contrast:

• Ja puhnem u dlanove, a on stavi rukavice.
• I blow into my palms, and he puts on gloves.

This is mainly about aspect and nuance.

• puhati → pušem = imperfective
• puhnuti → puhnem = perfective

Very roughly:

• pušem suggests blowing as an ongoing or repeated activity
• puhnem suggests a single, quick, completed puff

In this sentence, puhnem sounds like a natural quick reaction: it’s cold, so I give a puff into my palms.

If you said pušem u dlanove, that would also be possible, but it would feel more like the action is ongoing or more durative.

In a sentence like this, it is best understood as a habitual repeated action:

• Whenever it’s cold outside, I blow into my palms.

This can be tricky because perfective present forms in Croatian often do not behave exactly like ordinary English present tense.

Here, because of kad, the meaning is not really I am blowing right now, but rather this is what I do in that situation.

So the sentence describes a typical reaction, not an action happening at this exact moment.

Because the expression is puhnuti u dlanove.

A few things are happening here:

1. u is the normal preposition in this expression

Croatian says puhnuti u dlanove, even though English usually says blow on your hands.

This is a good reminder that prepositions often do not match exactly between languages.

2. dlanove is the accusative plural

The basic noun is:

• dlan = palm of the hand

Its plural is:

• dlanovi = palms

And here, after u, we get:

• dlanove = accusative plural

So:

• u dlanove = into the palms / into one’s palms

The dictionary form is dlan.

Useful forms:

• dlan = palm
• dlanovi = palms
• dlanove = accusative plural

In this sentence, plural is natural because people usually warm both hands.

If you were talking about only one palm, you could say u dlan, but that is less likely in this context.

Because Croatian often leaves out possessive words like my, your, or his when the owner is obvious, especially with body parts.

So:

• puhnem u dlanove

naturally means:

• I blow into my palms

Croatian does this very often:

• Perem ruke. = I’m washing my hands.
• Zatvorio je oči. = He closed his eyes.

If you say u svoje dlanove or u moje dlanove, that becomes more explicit and usually more emphatic than necessary.

Yes, the word order can change.

For example:

• Kad je vani hladno, puhnem u dlanove.
• Puhnem u dlanove kad je vani hladno.

Both are natural.

Why the comma in the original sentence?

The clause starting with kad comes first:

• Kad je vani hladno, = subordinate clause
• puhnem u dlanove. = main clause

When the subordinate clause comes first, Croatian normally separates it with a comma.

If you put the main clause first:

• Puhnem u dlanove kad je vani hladno.

the comma is usually omitted, because the kad-clause follows the main clause and is closely connected to it.