Kad budemo presjedali, provjerit ćemo s kojeg perona ide sljedeći vlak.

Because after conjunctions like kad, Croatian normally does not use the regular future with ću/ćeš/ćemo in this kind of subordinate clause.

Instead, Croatian usually uses:

• future II: kad budemo presjedali
• or, with some verbs, a present-form expression with future meaning

So:

• Kad budemo presjedali, provjerit ćemo... = When we are changing trains, we will check...

This is a very common pattern after words like kad, ako, čim when talking about the future.

Presjedati means to change trains / buses / transport, or more generally to transfer.

In this sentence, it means that we will be changing from one train to another.

Examples:

• presjedati na kolodvoru = to change trains at the station
• moramo presjedati u Zagrebu = we have to change trains in Zagreb

So Kad budemo presjedali means When we are making the transfer / when we are changing trains.

This is an aspect difference.

• presjedati = imperfective
• presjesti = perfective

Very roughly:

• presjedati focuses on the process, duration, or repeated nature
• presjesti focuses on one completed change

So:

• Kad budemo presjedali... = When we are changing trains...
• Kad presjednemo... = When we have changed / once we change trains...

In your sentence, presjedati makes sense because the checking happens during the transfer situation.

Because future II in Croatian is built with:

• a form of biti: budemo
• plus the l-participle of the main verb: presjedali

That participle agrees with the subject in number and sometimes gender.

Here:

• budemo presjedali = we will be changing
• presjedali is the masculine/mixed plural form

If the speakers were all female, you would hear:

• Kad budemo presjedale...

So the -li ending is part of Croatian agreement grammar.

In standard Croatian, when the future auxiliary comes after the infinitive, the final -i of the infinitive is dropped in writing.

So:

• provjeriti + ćemo becomes provjerit ćemo

Compare:

• Provjerit ćemo.
• Ćemo provjeriti.

Both mean the same thing, but the first one is the usual written form in this structure.

So this is a spelling-and-grammar rule of the future tense, not a change of meaning.

Because Croatian often leaves out subject pronouns when they are already clear from the verb ending.

Here:

• budemo = we will be
• provjerit ćemo = we will check

So we is already built into the verb forms.

You could add mi for emphasis:

• Kad budemo presjedali, mi ćemo provjeriti...

But in a neutral sentence, Croatian usually omits it.

This is because s/sa here means from, and with that meaning it takes the genitive case.

So:

• s = from
• kojeg = genitive of koji = which
• perona = genitive singular of peron

Literally:

• s kojeg perona = from which platform

That is exactly what English means in Which platform does the train leave from?

A useful contrast:

• na kojem peronu = on which platform
• s kojeg perona = from which platform

In this sentence, Croatian chooses the from which platform wording.

Yes, literally ide means goes, but in Croatian it is very normal to use ići for transport departures and routes.

So:

• Vlak ide s trećeg perona can mean The train goes/leaves from platform 3

In natural English, you would often translate this as:

• leaves from
• departs from

Croatian can also use polazi, which is a bit more formal or timetable-like:

• Vlak polazi s trećeg perona = The train departs from platform 3

So ide is not strange here at all.

Both are possible, but ide sljedeći vlak sounds very natural in this kind of embedded question.

The phrase:

• s kojeg perona ide sljedeći vlak

is an indirect-question structure, and Croatian often places the verb before the subject in that type of sentence.

So:

• s kojeg perona ide sljedeći vlak = natural
• s kojeg perona sljedeći vlak ide = also understandable, but less neutral here

Also, sljedeći vlak is still the subject, and it is in the nominative:

• sljedeći = nominative masculine singular
• vlak = nominative masculine singular

Yes. Peron is the normal everyday word for a train platform.

So:

• peron 1 = platform 1

The word platforma exists in Croatian, but it usually means a platform in other senses, such as:

• a physical platform/stage
• a technological platform
• an online platform

At a railway station, peron is the word learners should expect most often.

Because Kad budemo presjedali is a subordinate clause, and it comes before the main clause:

• Kad budemo presjedali, = subordinate clause
• provjerit ćemo s kojeg perona ide sljedeći vlak. = main clause

Croatian normally separates that fronted clause with a comma.

So the comma works much like in English:

• When we change trains, we’ll check...

It marks the boundary between the two parts of the sentence.