Learn Hardcore Croatian: More je na jugu, a planine su na sjeveru, dok se sunce ujutro diže na istoku. - The sea is in the south and the mountains are in the north, while the sun rises in the east in the morning.

There is no separate word for the; definiteness is understood from context. So More je na jugu = The sea is in the south (in this context).

Why is it More je, but planine su? Why two different forms of “to be”?

Because the verb biti (to be) agrees with the grammatical number of the subject.

more – neuter singular → take je (3rd person singular)
- More je na jugu. – The sea is in the south.
planine – feminine plural → take su (3rd person plural)
- Planine su na sjeveru. – The mountains are in the north.

So:

singular subject → je
plural subject → su

Why is planine in the plural? Could I say planina instead?

In Croatian, planine (plural) is the usual way to talk about mountains in a general, geographic sense:

Planine su na sjeveru. – (The) mountains are in the north.

You can say planina (singular – “a mountain” / “the mountain”), but then you’d be referring to one specific mountain:

Planina je na sjeveru. – The mountain is in the north.

In your sentence, the idea is “the mountains (mountain range)” in general, so the plural planine + plural verb su is natural.

What case are jugu, sjeveru, and istoku, and why do they all end in -u?

They are in the locative singular case:

jug → jugu (locative)
sjever → sjeveru (locative)
istok → istoku (locative)

The preposition na can take:

accusative – when you talk about movement to/onto somewhere
- Idem na jug. – I am going to the south.
locative – when you talk about being at/on a place
- More je na jugu. – The sea is in/on the south.

Here you’re describing location, not movement, so na + locative is used, and that gives the -u ending.

Why is the preposition na used with directions (na jugu, na sjeveru, na istoku) and not u?

With cardinal directions used as general regions on a map, Croatian typically uses na:

na jugu – in the south
na sjeveru – in the north
na istoku – in the east
na zapadu – in the west

u is more often used with enclosed places or countries/cities:

u kući – in the house
u Zagrebu – in Zagreb
u Hrvatskoj – in Croatia

There is some overlap (e.g. na moru “at the seaside”, na selu “in the countryside”), but for compass directions as large regions, na is standard.

Is the word order More je na jugu fixed, or can I say Na jugu je more?

Both are correct. Word order in Croatian is fairly flexible, but it affects emphasis:

More je na jugu. – neutral; “The sea is in the south.”
Na jugu je more. – emphasizes na jugu (in the south), like:
“It is in the south that the sea is.”

In your full sentence, other natural variants are:

Na jugu je more, a na sjeveru su planine, dok se sunce ujutro diže na istoku.
More je na jugu, a na sjeveru su planine…

Different orders are possible, but the given one is already natural and neutral.

What is the function of a in a planine su na sjeveru? How is it different from i?

Both a and i can connect clauses, but they have slightly different nuances:

i = “and” (simple addition)
a = “and / but / whereas” (mild contrast or opposition)

In More je na jugu, a planine su na sjeveru, a suggests a contrast:

The sea is in the south, whereas the mountains are in the north.

If you said More je na jugu, i planine su na sjeveru, it would sound more like a plain list, without highlighting the contrast. Native speakers strongly prefer a in this kind of contrasting pair.

What exactly does dok mean in dok se sunce ujutro diže na istoku?

dok usually means while, sometimes whereas:

Dok učim, ne slušam glazbu. – While I study, I don’t listen to music.
On radi, dok ona spava. – He works, while she sleeps.

In your sentence, dok introduces another event happening in time:

…dok se sunce ujutro diže na istoku.
– …while the sun rises in the east in the morning.

So the whole sentence is:

The sea is in the south, and the mountains are in the north, while the sun rises in the east in the morning.

Why do we need se in dok se sunce ujutro diže? What does se add?

se is a clitic that often marks reflexive or intransitive use of a verb. With dizati se, it forms the meaning to rise / to get up:

dizati – to lift (something)
dizati se – to rise (oneself), to get up, for the sun: to rise

Without se, dizati is more like “to raise / to lift (something)”:

On diže teret. – He is lifting a load.
Sunce se diže. – The sun is rising.

So se is necessary here to get the correct intransitive meaning “rise” for the sun.

Where does se normally go in the sentence? Is dok se sunce ujutro diže the only correct order?

In Croatian, se is a clitic and tends to stand in the second position in a clause (after the first stressed word or phrase). All of these are possible and natural:

Dok se sunce ujutro diže na istoku.
Dok se sunce diže ujutro na istoku.
Sunce se ujutro diže na istoku.
Sunce se diže ujutro na istoku.

But you cannot put se far away from the verb or at the very end:

✗ Dok sunce ujutro diže se na istoku. (wrong)

So the given order dok se sunce ujutro diže na istoku is natural and follows the clitic‑second rule.

What does ujutro mean, and can I put it in a different place, like Sunce se diže ujutro na istoku?

ujutro means in the morning. It’s an adverb of time and is fairly flexible in position:

Sunce se ujutro diže na istoku.
Sunce se diže ujutro na istoku.
Ujutro se sunce diže na istoku.

All are correct. The differences are minor and mostly about rhythm and slight emphasis. Placing ujutro after the subject (sunce) is very common and sounds natural, as in the original sentence.

Is there a difference between sunce se diže and sunce izlazi for “the sun rises”?

Both are used, but with slightly different flavors:

sunce se diže – literally “the sun is rising / lifts itself”, more image-like or descriptive.
sunce izlazi – literally “the sun comes out / goes out”, very common, maybe the most standard way to say the sun rises.

You could naturally say:

More je na jugu, a planine su na sjeveru, dok sunce ujutro izlazi na istoku.

That version is just as idiomatic as the one with se diže.

Why is there a comma before a and dok in this sentence?

Croatian punctuation uses commas between main clauses and also before many conjunctions introducing a new clause:

More je na jugu, a planine su na sjeveru, dok se sunce ujutro diže na istoku.

Breakdown:

More je na jugu – main clause
a planine su na sjeveru – another main clause, introduced by a → comma before a
dok se sunce ujutro diže na istoku – subordinate clause of time, introduced by dok → comma before dok

So commas mark the boundaries between these clauses, very similar to many English comma rules before while, whereas, but, etc.

Why aren’t jugu, sjeveru, and istoku capitalized like English “South”, “North”, “East”?

In Croatian, cardinal directions used in a geographic sense are normally not capitalized:

jug, sjever, istok, zapad – south, north, east, west
na jugu Europe – in the south of Europe
na sjeveru zemlje – in the north of the country

They are capitalized only when part of proper names (rare with directions), for example if in an official name. But in your sentence, they are just common nouns, so lowercase is correct: na jugu, na sjeveru, na istoku.