Ako vlak bude kasnio, pozvat ću te s kolodvora.

Because after ako when you are talking about a future condition, Croatian normally uses future II, not future I.

So the pattern is:

• Ako + future II, main clause in future I

That is exactly what you have here:

• Ako vlak bude kasnio = if the train is late / if the train ends up running late
• pozvat ću te = I will call you

For an English speaker, this is a bit like learning that Croatian handles future if-clauses differently from English. English says If the train is late, I’ll call you. Croatian does not use će kasniti in that clause.

It is future II.

It is formed with:

• budem, budeš, bude, budemo, budete, budu
• plus the l-participle of the main verb

Here:

• bude = 3rd person singular
• kasnio = l-participle of kasniti

So:

• bude kasnio = future II, 3rd person singular masculine

Do not try to translate it too literally word for word. In this sentence, it simply functions as the normal Croatian form after ako for a future condition.

Because the participle agrees with the subject in gender and number.

The subject is:

• vlak = train

Vlak is masculine singular, so the participle must also be masculine singular:

• kasnio

Compare:

• Ako pošta bude kasnila... because pošta is feminine singular
• Ako vlakovi budu kasnili... because vlakovi is plural

Both are correct.

Croatian future I is made with the clitic forms of htjeti plus the infinitive. You can place them in two common ways:

• ću pozvati
• pozvat ću

When the infinitive comes before the clitic, the final -i of the infinitive is often dropped:

• pozvati
  • ću → pozvat ću

So:

• Pozvat ću te = correct
• Ja ću te pozvati = also correct

There is no major difference in meaning here.

Because pozvati takes a direct object, and that object is in the accusative.

Here, te is the unstressed accusative clitic meaning you.

So:

• pozvati te = to call you

Useful comparison:

• te = unstressed accusative/genitive clitic
• tebe = full/emphatic accusative form
• ti = usually nominative you or dative clitic, so it does not fit here

So pozvat ću te means I’ll call you, not I’ll call to you.

Because they are clitics, and Croatian clitics usually go in the second position in the clause.

In this sentence:

• pozvat is the first stressed element
• then come the clitics ću te

So you get:

• Pozvat ću te s kolodvora

This word order is very normal in Croatian.

You could also say:

• Ja ću te pozvati s kolodvora

There, ja is the first element, and the clitics follow it.

Both can mean from, but the nuance is different.

• s kolodvora usually means from the station, in the sense of the station area, platforms, or the place generally
• iz kolodvora more strongly suggests out of the inside of the station building

In this sentence, s kolodvora is the natural idiomatic choice for I’ll call you from the station.

It is genitive singular.

The base form is:

• kolodvor = station

After the preposition s meaning from, Croatian uses the genitive:

• s kolodvora = from the station

So:

• nominative: kolodvor
• genitive: kolodvora

In standard Croatian:

• kolodvor usually means a larger station/terminal, especially for trains or buses
• stanica is a more general stop/station

Typical examples:

• željeznički kolodvor = railway station
• autobusni kolodvor = bus station
• tramvajska stanica = tram stop/station
• autobusna stanica = bus stop

Since the sentence is about a train, kolodvor is the natural word.

Yes, in this context it does.

Pozvati can mean several related things, such as:

• to call
• to invite
• to summon

But in a sentence like this, with te and s kolodvora, the intended meaning is clearly to call you on the phone.

A more specifically phone-related verb is nazvati. So you could also say:

• Nazvat ću te s kolodvora.

That would sound very natural too.

Because Croatian does not have articles like a and the.

So:

• vlak can mean a train or the train
• kolodvor can mean a station or the station

The exact meaning comes from context.

In this sentence, English naturally uses the:

• If the train is late, I’ll call you from the station.

But Croatian simply uses the noun without any article.

Because Ako vlak bude kasnio is a subordinate clause, and it comes before the main clause.

So Croatian writes:

• Ako vlak bude kasnio, pozvat ću te s kolodvora.

That comma separates:

• the if-clause
• the main clause

This is standard punctuation in Croatian.