Puhni u juhu ako je prevruća.

Puhni is the 2nd person singular imperative form, so it is a command or instruction said to one person in an informal you form.

It comes from the verb puhnuti.

So grammatically, it means something like:

• blow!
• you, blow!

If you were speaking to more than one person, or using the polite/formal you, you would say puhnite.

Because Croatian and English do not always use the same preposition with the same verb.

With puhati / puhnuti, Croatian normally uses u:

• puhnuti u juhu = blow on/into the soup
• puhnuti u svijeću = blow on/out a candle

So this is best learned as a fixed verb pattern:

• puhnuti u + accusative

Even though English says blow on, Croatian says puhnuti u.

Because after u in this sentence, the noun is in the accusative case.

The noun juha is feminine singular, and its accusative singular form is juhu.

So:

• nominative: juha
• accusative: juhu

That is why the sentence has u juhu, not u juha.

Croatian often leaves out the subject when it is already clear from context.

Here, the understood subject is juha. So:

• ako je prevruća literally means if it is too hot
• but the it is not said explicitly, because Croatian usually does not need it

English often needs a subject like it, but Croatian can simply say je prevruća because the listener already knows we are talking about the soup.

The prefix pre- often means too or overly.

So:

• vruća = hot
• prevruća = too hot

In this sentence, prevruća means the soup is not just hot, but more hot than is comfortable or suitable.

Because it agrees with juha, which is a feminine singular noun.

Croatian adjectives must agree with the noun they describe in:

• gender
• number
• case

Since juha is feminine singular, the adjective is also feminine singular:

• juha je prevruća

If the noun were masculine or neuter, the adjective would change:

• čaj je prevruć = the tea is too hot
• mlijeko je prevruće = the milk is too hot

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

So:

• je = is

In ako je prevruća, it means if it is too hot.

Even though Croatian can omit the subject, it still needs the verb, so je stays.

In a sentence like this, Croatian often does not use a comma before a short subordinate clause introduced by ako when it comes after the main clause.

So this is normal:

• Puhni u juhu ako je prevruća.

If the ako clause comes first, then you normally put a comma after it:

• Ako je prevruća, puhni u juhu.

So both word orders are possible, but punctuation changes with them.

Yes. Croatian word order is fairly flexible.

You could also say:

• Ako je juha prevruća, puhni u juhu.

That puts the if clause first, which can sound a bit more like setting the condition before giving the instruction.

The original order is also perfectly natural:

• Puhni u juhu ako je prevruća.

That starts with the command and then adds the condition.

Yes, there is a small aspect difference.

• puhni comes from puhnuti and is perfective
• puhaj comes from puhati and is imperfective

In simple terms:

• puhni sounds like give it a blow / blow once / do the action
• puhaj sounds more like keep blowing / blow for a while

In this kind of everyday instruction, puhni is very natural.
If you wanted to stress a more continuous action, puhaj could also make sense.

You would use the plural/formal imperative:

• Puhnite u juhu ako je prevruća.

So:

• puhni = singular informal
• puhnite = plural or polite/formal

This is the same distinction Croatian often makes with commands.