Learn Hardcore Croatian: Na planini je zrak hladan, ali pogled na more s otoka je topao u srcu. - In the mountains the air is cold, but the view of the sea from the island is warm in the heart.

Croatian allows some flexibility in word order for emphasis. Putting na planini first emphasizes the location. Je (is) usually stays in the second position in the clause, so we get Na planini je zrak hladan.

Could it also be Zrak je hladan na planini? Is there a difference?

Yes, that is correct too:

Zrak je hladan na planini.

Meaning is essentially the same: The air is cold on the mountain.

Subtle nuance:

Na planini je zrak hladan – starts by setting the scene (On the mountain…).
Zrak je hladan na planini – starts by talking about the air, then specifies where it is cold.

Both are natural and grammatically correct.

Why do we use ali and not a?

Both ali and a can be translated as but, but they are used a bit differently.

ali = but, usually for a stronger, clearer contrast, often like however.
a = can mean and / but / while, often for a weaker contrast or just switching to a different aspect or perspective.

In:

Na planini je zrak hladan, ali pogled na more s otoka je topao u srcu.

we have a strong, emotional contrast: the air is cold, but the view is warm (in the heart). That makes ali the more natural choice. Using a would sound a bit softer and more like a simple contrast of two scenes.

Why is it pogled na more and not pogled na moru?

Because pogled na (view of / view at) takes the accusative case, not the locative.

more = nominative/accusative singular (neuter)
moru = locative singular

Compare:

pogled na more – a view of the sea (grammatical pattern after pogled na)
na moru – on the sea, at the seaside (locative, static location)

So pogled na moru would be wrong here; na more is required by the expression pogled na što (a view of something).

Why is more not changed in form? It looks the same as the dictionary form.

More (sea) is a neuter noun, and in the singular:

nominative = accusative

So:

(to) je more – that is the sea (nominative)
vidim more – I see the sea (accusative)
pogled na more – a view of the sea (accusative)

The form happens to be identical in these two cases, which is why it looks unchanged.

What case is otoka in s otoka, and why do we use s?

Otoka is in the genitive case. After the preposition s meaning from, we use the genitive:

s otoka – from the island (preposition s
- genitive)

So:

otok = island (nominative)
s otoka = from the island (genitive after s)

The preposition s here means from (a surface or place), not with. When it means from, it is always followed by the genitive.

What is the difference between s otoka, sa otoka, and iz otoka?

s otoka – standard and very common: from the island.
sa otoka – also used, often for ease of pronunciation; in this case s otoka and sa otoka mean the same thing.
iz otoka – generally wrong in standard Croatian for this meaning; iz (from, out of) is usually used with enclosed spaces (e.g., iz kuće – out of the house).

So for from the island (as a place you stand on) you use s otoka (or sa otoka).

Why is it u srcu and not na srcu?

U means in, inside, and srce (heart) is seen as something you are in, not on.

u srcu – in (the) heart → literally: inside the heart
na srcu – can exist, but has a more concrete or idiomatic sense like on the heart (e.g. leži mi nešto na srcu – something weighs on my heart).

In topao u srcu, the idea is that the feeling is inside your heart, so u + locative (srcu) is used.

Why is it srcu and not srce?

Because srcu is the locative singular of srce:

srce – heart (nominative)
u srcu – in the heart (locative after u for location)

The preposition u meaning in/inside (somewhere) takes the locative case when there is no movement. So we must say u srcu.

Why topao u srcu and not toplo u srcu?

Both forms are possible, but they are slightly different grammatically:

topao – adjective, masculine singular, agreeing with pogled (view).
- pogled … je topao u srcu – the view is warm in (the) heart.
toplo – adverb: warmly.
- osjećam se toplo u srcu – I feel warm in my heart.

In the given sentence, topao describes pogled (the view), so it needs the adjective form that matches pogled (masculine singular):

pogled (m.) je topao
zrak (m.) je hladan

If you changed the subject, you might use toplo:

U srcu mi je toplo. – My heart feels warm.

What cases are used in the whole sentence, and which prepositions trigger them?

The sentence:

Na planini je zrak hladan, ali pogled na more s otoka je topao u srcu.

Cases and prepositions:

na planini – locative (planini), after na (location = on/at)
zrak – nominative (subject)
pogled – nominative (subject of the second clause)
na more – accusative (more), pattern pogled na + accusative
s otoka – genitive (otoka), after s = from
u srcu – locative (srcu), after u = in (location)

So we have: nominative, genitive, accusative, and locative in this one sentence.

Croatian has no articles. How do we know it means “the air” and “the sea” and not “an air” or “a sea”?

Croatian does not have articles (a, an, the). Whether you understand something as a/an or the comes from context, not from a separate word.

zrak can mean air / the air
more can mean sea / the sea

Here, we are clearly talking about specific, real things:

zrak on the mountain → the air (there)
more s otoka → the sea (seen from that island)

So in natural English we translate this as:

On the mountain the air is cold, but the view of the sea from the island is warm in the heart.