Danas je vani trideset stupnjeva.

• Danas = today
• je = is
• vani = outside / outdoors
• trideset = thirty
• stupnjeva = degrees

So grammatically it is something like Today, it is outside thirty degrees, but in natural English that becomes Today it is thirty degrees outside.

Croatian often uses impersonal expressions for weather, time, and similar situations. English needs a dummy subject like it in It is hot or It is thirty degrees, but Croatian does not.

So je is simply the 3rd person singular of biti (to be), and the sentence works without an explicit subject.

Because after numbers like 5 and above, Croatian normally uses the genitive plural form of the noun.

• 5 stupnjeva
• 10 stupnjeva
• 30 stupnjeva

The basic plural stupnjevi is not used after trideset here.

The form changes depending on the number:

• jedan stupanj = one degree
• dva stupnja = two degrees
• tri stupnja = three degrees
• četiri stupnja = four degrees
• pet stupnjeva = five degrees
• trideset stupnjeva = thirty degrees

A useful shortcut:

• 1 → singular
• 2, 3, 4 → usually stupnja
• 5+ → stupnjeva

This pattern is very common in Croatian.

Vani means outside / outdoors and is the natural choice in this sentence.

• Danas je vani trideset stupnjeva. = Today it is thirty degrees outside.

Van can also relate to outside, but it is often used differently, especially with movement or in set expressions. For a weather sentence, vani sounds more natural.

Yes. You can say:

• Danas je trideset stupnjeva.

That still means Today it is thirty degrees. In many situations, outside is understood anyway.

Adding vani makes it more explicit that you mean the outdoor temperature.

Yes. You can say:

• Vani je trideset stupnjeva.

That means It is thirty degrees outside.

So:

• danas adds the time idea: today
• vani adds the location idea: outside

Not completely. Croatian word order is more flexible than English, but je is a clitic, so it usually comes very early in the sentence, often in second position.

For example, these are possible:

• Danas je vani trideset stupnjeva.
• Vani je danas trideset stupnjeva.

Both are natural, with slightly different emphasis.

What usually sounds wrong is placing je too late.

Because in everyday Croatian, just saying stupnjeva is often enough when the scale is obvious from context.

If needed, you can make it explicit:

• trideset stupnjeva Celzija

But in normal conversation, trideset stupnjeva is very common and natural.

A helpful approximation is:

STOOP-nyeh-va

The important part is nj, which is a single Croatian sound, like the ny in canyon.

So:

• stu → like stoo
• pnje → roughly p-nye
• va → va

You do not pronounce n and j as two completely separate sounds.

Yes, absolutely. It sounds normal and idiomatic.

Croatian speakers commonly use this kind of structure for temperature:

• Vani je deset stupnjeva.
• Danas je dvadeset pet stupnjeva.
• Jučer je bilo trideset stupnjeva.

So Danas je vani trideset stupnjeva is a perfectly natural sentence.