Za roštilj nam treba još ugljena.

A very literal breakdown is:

• za roštilj = for the barbecue / for grilling
• nam = to us
• treba = is needed / is necessary
• još ugljena = more charcoal

So the sentence is built more like:

For the barbecue, to us, more charcoal is needed.

Natural English is simply:

We need more charcoal for the barbecue.

Za here means for.

After za, Croatian usually uses the accusative case. So roštilj is in the accusative singular.

Because roštilj is a masculine inanimate noun, its nominative and accusative singular are the same:

• nominative: roštilj
• accusative: roštilj

So even though the form does not change, the case is still accusative because of za.

Roštilj can mean:

• a grill
• barbecue
• a barbecue meal/event, depending on context

In this sentence, because of ugljena (charcoal), za roštilj most naturally means:

• for the barbecue
• or for grilling

So it is not just the metal object; it can also refer to the whole grilling situation.

Nam is the dative form of mi and means to us.

In Croatian, with trebati, the person who needs something is often put in the dative:

• Treba mi vode. = I need water.
  Literally: Water is needed to me.
• Treba nam još ugljena. = We need more charcoal.
  Literally: More charcoal is needed to us.

So nam tells you who needs it.

Because this sentence uses the very common dative + treba pattern.

Croatian often expresses need like this:

• Treba mi... = I need...
• Treba nam... = We need...
• Treba mu... = He needs...

So treba does not mean only he/she/it needs here. In this structure, it works more like is needed.

You can also say:

Trebamo još ugljena za roštilj.

That also means We need more charcoal for the barbecue. But the version with nam treba is extremely common and very natural.

Because after words that express an amount or quantity, Croatian often uses the genitive.

Here, još means more / some more, and that triggers the genitive:

• još ugljena = more charcoal

The base form is:

• ugljen = charcoal / coal

The form in the sentence is:

• ugljena = genitive singular

This is similar to other quantity expressions:

• puno vode = a lot of water
• malo vremena = a little time
• još kruha = more bread

Here još means more or some more.

So:

• još ugljena = more charcoal

In other contexts, još can also mean still:

• Još čekam. = I am still waiting.

But in this sentence, because it comes before a noun expressing quantity, the meaning is clearly more.

Yes, Croatian word order is flexible.

The sentence:

Za roštilj nam treba još ugljena

is natural because it puts za roštilj first for context: as for the barbecue...

You could also say:

• Treba nam još ugljena za roštilj.
• Još ugljena nam treba za roštilj.

All of these are understandable, but they shift the emphasis a little.

One important thing: nam is a clitic, so it usually appears near the beginning of the clause, in an early position.

In this sentence, treba is normal because the construction is not treating ugljena as a regular nominative subject.

Instead, Croatian is using an impersonal/amount-style construction:

• Treba nam još ugljena.

That is why singular treba is used.

If you had a more straightforward nominative plural subject, you could get forms like:

• Trebaju nam stolice. = We need chairs.

But with a mass noun and a quantity expression like još ugljena, treba is the expected form.

Yes, absolutely.

Both are natural:

• Za roštilj nam treba još ugljena.
• Trebamo još ugljena za roštilj.

The difference is mostly in structure and style:

• nam treba sounds like more charcoal is needed for us
• trebamo is more directly we need

The first version is especially common in everyday Croatian.

They mean different things:

• za roštilj = for the barbecue / for grilling
• na roštilju = at the barbecue / on the grill

Examples:

• Trebamo meso za roštilj. = We need meat for the barbecue.
• Meso je na roštilju. = The meat is on the grill.
• Vidimo se na roštilju. = See you at the barbecue.

So in your sentence, za roštilj means purpose, not location.

It is very natural, everyday Croatian.

It sounds like something someone would really say while preparing food or shopping:

• Za roštilj nam treba još ugljena.

It is neither overly formal nor slangy. It is a completely normal conversational sentence.