Treba podgrijati juhu prije nego što gosti dođu.

In this sentence, treba is being used impersonally. It means something like:

• it is necessary
• one should
• we need to

So Treba podgrijati juhu means The soup needs to be reheated or We should reheat the soup.

There is no stated subject here. Croatian often uses treba this way when the speaker is talking about a general necessity, without saying exactly who must do it.

Because treba is followed by an infinitive to express what needs to be done.

So the structure is:

• treba + infinitive

Examples:

• Treba ići. = One should go.
• Treba kupiti kruh. = Bread needs to be bought.
• Treba podgrijati juhu. = The soup needs to be reheated.

This is very common in Croatian.

Because juha is the direct object of podgrijati.

The dictionary form is juha = soup.
But as a direct object, it goes into the accusative singular, which is juhu.

So:

• juha = nominative
• juhu = accusative

This is a very common pattern for many feminine nouns ending in -a:

• voda → vodu
• kava → kavu
• juha → juhu

Grijati means to heat / warm in a general sense.
Podgrijati means to reheat / warm up.

The prefix pod- often gives the idea of heating again or warming something up, especially food.

So:

• grijati juhu = to heat the soup
• podgrijati juhu = to reheat the soup

In this sentence, podgrijati is used because the soup is probably already cooked and just needs warming before the guests arrive.

Because podgrijati is perfective, while podgrijavati is imperfective.

Here, the speaker is talking about a single completed action: the soup should be reheated before the guests arrive. That makes the perfective verb the natural choice.

• podgrijati = to reheat once, to completion
• podgrijavati = to be reheating / to reheat repeatedly or as a process

So Treba podgrijati juhu focuses on getting the action done.

Prije nego što means before or more literally before ...

It introduces a subordinate clause:

• prije nego što gosti dođu = before the guests arrive

You can think of it as a set phrase. It connects the main action with another action that happens later.

Main clause:

• Treba podgrijati juhu = The soup needs to be reheated

Subordinate clause:

• prije nego što gosti dođu = before the guests arrive

Together:

• Treba podgrijati juhu prije nego što gosti dođu.

Because dođu is from the perfective verb doći (to come / to arrive), and Croatian often uses the present tense of a perfective verb to refer to a future completed event, especially after words like before, when, after, etc.

So:

• doći = perfective, to arrive
• dođu = present form, but here it refers to a future event: when they arrive / before they arrive

If you said gosti dolaze, that would usually mean the guests are coming / the guests come, focusing more on the ongoing event or general fact.

In this sentence, the idea is:

• first the soup gets reheated
• then the guests arrive

That is why dođu fits well.

Gosti is nominative plural because it is the subject of dođu.

Singular:

• gost = guest

Plural:

• gosti = guests

So:

• gosti dođu = the guests arrive

Since gosti is doing the action, it stays in the nominative.

Yes, in many contexts you can hear or see prije nego without što.

So both can occur:

• prije nego što gosti dođu
• prije nego gosti dođu

The version with što is very common and often feels a bit more complete or standard in careful speech and writing. As a learner, using prije nego što is a safe choice.

Yes. Croatian word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The neutral version is:

• Treba podgrijati juhu prije nego što gosti dođu.

But you could also say:

• Juhu treba podgrijati prije nego što gosti dođu.
• Prije nego što gosti dođu, treba podgrijati juhu.

These all mean basically the same thing, but the emphasis changes slightly:

• starting with juhu emphasizes the soup
• starting with prije nego što... emphasizes the timing

Both can express necessity, but they are not exactly the same.

• trebati often means it is necessary / should / need to
• morati is usually stronger: must / have to

So:

• Treba podgrijati juhu = The soup should be reheated / needs reheating
• Mora se podgrijati juha = The soup must be reheated

In your sentence, treba sounds natural and slightly less forceful. It suggests a practical necessity rather than a strict obligation.