Learn Hardcore Croatian: Kad putujemo prema zapadu, zalazak sunca izgleda duže. - When we travel toward the west, the sunset seems longer.

kad – shorter, more informal, very common in spoken and everyday written Croatian.
kada – a bit more formal or emphatic, common in careful speech and writing.

You can say:

Kad putujemo prema zapadu, …
Kada putujemo prema zapadu, …

Both are correct and mean the same. In many sentences they are completely interchangeable.

Why is the present tense putujemo used, even though this talks about a general fact?

Croatian, like English, uses the present tense for general truths or habitual situations.

Kad putujemo prema zapadu, zalazak sunca izgleda duže.
= When(ever) we travel westward, the sunset seems longer.

This is not about one specific trip but about what generally happens in that situation, so the simple present putujemo is exactly what you want.

You could say something like kad budemo putovali, but that would sound like talking about a particular future trip, not a general observation.

What does prema mean here, and why is it prema zapadu and not something like na zapad?

prema means towards / in the direction of and it always takes the dative case.

prema zapadu – towards the west, in a westward direction

na zapad (with accusative) is also possible in other contexts and feels more like to the West (as a destination), for example:

Idemo na zapad. – We are going to the West.

In your sentence, prema zapadu nicely emphasizes the direction of travel rather than a destination country/region called “the West”.

Why is it zapadu and not zapad or zapada after prema?

Because prema is a preposition that always requires the dative case.

The singular forms of zapad (west) are:

Nominative: zapad (as subject)
Genitive: zapada
Dative: zapadu
Accusative: zapad
Instrumental: zapadom

So:

prema + dative → prema zapadu

That is why the form zapadu is used here.

What exactly does zalazak sunca mean grammatically?

zalazak sunca literally means the setting of the sun and functions as sunset.

It is made of:

zalazak – a noun meaning setting, going down (from the verb zalaziti / zaći – to set, to go down)
sunca – genitive singular of sunce (the sun)

So zalazak sunca = “the sun’s setting” → sunset.

You can also sometimes see just zalazak if it’s clear from context that you mean sunset, but zalazak sunca is the more explicit, standard expression.

Why is there no mi before putujemo? How do we know the subject is “we”?

Croatian is a pro‑drop language: subject pronouns are often omitted because the verb ending already shows the person and number.

putujem – I travel
putuješ – you (sg) travel
putuje – he/she/it travels
putujemo – we travel
putujete – you (pl) travel
putuju – they travel

So putujemo already includes the meaning we. You would only add mi for emphasis or contrast:

Mi putujemo prema zapadu, a oni prema istoku.
We are travelling west, and they east.

Can I change the word order to put the main clause first, like Zalazak sunca izgleda duže kad putujemo prema zapadu?

Yes, that word order is completely natural:

Kad putujemo prema zapadu, zalazak sunca izgleda duže.
Zalazak sunca izgleda duže kad putujemo prema zapadu.

Both are correct and mean the same thing.

About the comma:

When the kad/kada clause comes first, you must put a comma:
Kad putujemo prema zapadu, zalazak sunca izgleda duže.
When it comes second, the comma is often omitted in modern usage because the sentence is short and clear:
Zalazak sunca izgleda duže kad putujemo prema zapadu.

You may still sometimes see the comma kept in both positions, but it’s not required when the time clause follows.

Why is it duže here? What about dulje or duži?

In this sentence, duže is the comparative of the adverb dugo (for a long time). It means longer (in time) or for a longer time.

dugo – for a long time
duže (or in standard Croatian often dulje) – for a longer time

So:

zalazak sunca izgleda duže
≈ the sunset seems longer (in duration)

A few points:

duže vs. dulje
- In Croatian you will often see dulje as the standard comparative.
- duže is also very widely used and understood; many speakers use it naturally.
  In everyday communication, both are fine; if you want to be closer to the standard, you can remember dugo → dulje.
Why not duži?
- duži is the masculine comparative adjective form of dug (long in space, or as an adjective in general), e.g.
  dug film → duži film (a long film → a longer film)
- Here we are talking about how long something lasts, so the adverbial form (duže/dulje) is more natural than the adjective duži.

Why is it izgleda duže and not something like traje duže? Is izgleda the best verb here?

izgleda comes from izgledati and means to look / to seem / to appear.

zalazak sunca izgleda duže
– the sunset seems / appears longer (to us)

This focuses on our perception: it seems longer, even if the actual duration may not change much.

If you want to talk about actual duration, you could indeed use trajati:

Kad putujemo prema zapadu, čini se da zalazak sunca traje duže.
– When we travel west, it seems that the sunset lasts longer.

So:

izgleda duže – it looks / seems longer (subjective impression)
traje duže – it lasts longer (objective duration)

Could we use dok instead of kad: Dok putujemo prema zapadu, zalazak sunca izgleda duže? Would that change the meaning?

You can use dok, but there is a slight nuance:

kad/kada – often when(ever), can refer to a single moment or a situation; here it expresses a general condition:
Kad putujemo prema zapadu, zalazak sunca izgleda duže.
– When(ever) we travel west, the sunset seems longer.
dok – usually means while, focusing more on something happening at the same time as another ongoing action:
Dok putujemo prema zapadu, zalazak sunca izgleda duže.
– While we are travelling west, the sunset seems longer.

In this particular sentence, both are understandable and similar in meaning, but:

kad sounds like a general rule or observation.
dok makes you picture more strongly the process of travelling and the sunset happening during that process.

For a general “when(ever)” statement like yours, kad/kada is the more typical choice.