Kad presjedaš, važno je znati s kojeg perona ide sljedeći vlak.

Kad means when. In this sentence, it introduces a time clause: Kad presjedaš = When you change trains / when you transfer.

Yes, kad and kada usually mean the same thing. The difference is mostly style:

• kad = more common in everyday speech
• kada = a bit more formal or emphatic

So you could also say:

Kada presjedaš, važno je znati s kojeg perona ide sljedeći vlak.

Both are natural.

Presjedaš comes from the verb presjedati / presjesti, which means to change trains/buses/trams, to transfer, or more literally to switch from one vehicle to another.

In travel contexts, it is very common. For example:

• Presjedam u Zagrebu. = I’m changing trains/buses in Zagreb.
• Moraš presjesti na drugi vlak. = You have to change to another train.

In your sentence, presjedaš means you are transferring / you change trains.

Croatian often uses the present tense for things that are:

• general truths
• repeated situations
• instructions
• things that happen whenever a condition is met

So Kad presjedaš... does not necessarily mean when you are transferring right now. It often means:

• when you transfer
• whenever you transfer
• when changing trains

This is very normal in Croatian. English often uses a similar present tense too:

• When you travel, it’s important to...
• When you transfer, you need to know...

Važno je znati literally means It is important to know.

This is an impersonal structure:

• važno = important
• je = is
• znati = to know

Croatian often uses this pattern:

• Dobro je jesti zdravo. = It is good to eat healthy food.
• Teško je učiti noću. = It is hard to study at night.
• Važno je znati... = It is important to know...

So the sentence is built like this:

When you transfer, it is important to know...

Because after važno je, Croatian commonly uses the infinitive to express a general idea:

• Važno je znati. = It is important to know.
• Važno je razumjeti. = It is important to understand.

If you said važno je da znaš, that would also be possible, but it is a different structure:

• Važno je znati s kojeg perona ide sljedeći vlak.
  = It is important to know which platform the next train leaves from.
• Važno je da znaš s kojeg perona ide sljedeći vlak.
  = It is important that you know which platform the next train leaves from.

Both work, but the infinitive version sounds very natural and general.

S kojeg perona means from which platform.

Breakdown:

• s / sa = from
• kojeg = which in the genitive singular
• perona = platform in the genitive singular

The noun peron is masculine, and after s/sa meaning from, Croatian uses the genitive:

• peron → nominative
• s perona = from the platform
• s kojeg perona = from which platform

So the sentence is asking about the point of departure:
which platform the next train goes/leaves from.

Because koji changes form depending on case, gender, and number.

Here, it follows s in the sense of from, which requires the genitive. Since peron is masculine singular, the correct form is kojeg.

A small pattern:

• koji peron = which platform (nominative)
• s kojeg perona = from which platform (genitive)
• na kojem peronu = on which platform (locative)

So kojeg is simply the case form required by the preposition and noun.

Yes, but the meaning shifts a little.

• s kojeg perona ide vlak = which platform the train goes/leaves from
• na kojem peronu je vlak = which platform the train is on
• na kojem peronu staje vlak = which platform the train stops at / is arriving at

In your sentence, s kojeg perona is very natural because the focus is departure: you need to know from which platform the next train leaves.

English often says what platform does it go from?, and Croatian matches that idea well with s kojeg perona.

Yes, ide literally means goes, but in Croatian it is very commonly used for vehicles, routes, and departures.

So:

• Vlak ide s trećeg perona. = The train goes/leaves from platform 3.
• Autobus ide u 8. = The bus leaves/goes at 8.
• Ovaj vlak ide za Split. = This train goes to Split.

In English, goes can sound too literal in some transport sentences, but in Croatian ići is very normal here.

Sljedeći vlak is in the nominative singular, because it is the subject of ide.

• sljedeći = next (masculine singular nominative)
• vlak = train (masculine singular nominative)

So:

sljedeći vlak ide = the next train goes / leaves

If the form changed case, it would look different, for example:

• vidim sljedeći vlak = accusative, though for this masculine inanimate noun it looks the same
• bez sljedećeg vlaka = genitive

Croatian word order is fairly flexible, but this version is very natural:

Kad presjedaš, važno je znati s kojeg perona ide sljedeći vlak.

It flows as:

1. time condition: Kad presjedaš
2. main statement: važno je znati
3. embedded question: s kojeg perona ide sljedeći vlak

You could rearrange parts for emphasis, but not every rearrangement sounds equally natural. For example:

• Važno je, kad presjedaš, znati s kojeg perona ide sljedeći vlak.
  This is possible, but slightly more marked.

The original order is probably the most neutral and idiomatic.

It is the most natural and standard word here.

• sljedeći vlak = the next train

Croatian also has idući, which can also mean next in many contexts:

• idući vlak

In everyday speech, both may be heard, but sljedeći vlak is very standard and widely acceptable.

So for learners, sljedeći vlak is a very safe choice.