Ako budemo morali dugo čekati na peronu, otići ćemo u čekaonicu.

Because after ako when you are talking about a future possibility, Croatian normally uses the future II in the if-clause, not the ordinary future I.

So:

• Ako budemo morali... = If we have to / if it turns out that we will have to...
• ...otići ćemo... = ...we will go...

This is a very common pattern:

• Ako budem imao vremena, doći ću.
  If I have time / if I happen to have time, I’ll come.

So the sentence has:

• future II in the subordinate clause: budemo morali
• future I in the main clause: otići ćemo

Using ako ćemo morati would sound unnatural in standard Croatian in this kind of sentence.

Budemo is a form of the verb biti (to be).

More specifically, it is the 1st person plural form used in the future II construction:

• ja budem
• ti budeš
• on/ona/ono bude
• mi budemo
• vi budete
• oni/one/ona budu

In this sentence, budemo goes with mi (we), even though mi is not stated explicitly.

So:

• budemo morali = we will have to / if we have to

Because in future II, the modal verb morati appears as its past participle:

• infinitive: morati = to have to
• past participle (masculine plural): morali

So:

• budemo morali is the correct future II form
• not budemo morati

This is similar to other compound tenses in Croatian, where you combine an auxiliary with a participle.

Because the subject is we.

Even though Croatian often leaves the pronoun out, the verb form tells you the subject:

• budemo = we

Since the understood subject is mi, the participle also agrees with that subject, so it becomes plural:

• mi budemo morali

If the subject were singular, it would change:

• Ako budem morao... = If I have to... (masculine speaker)
• Ako budem morala... = If I have to... (feminine speaker)

For we, if the group is mixed or unspecified, Croatian normally uses the masculine plural form morali.

Because after the modal verb morati, the next verb normally stays in the infinitive.

So:

• morati čekati = to have to wait
• budemo morali čekati = we will have to wait

This is the same pattern as in English:

• we have to wait
• we will have to wait

The modal verb carries the tense, and the main action verb stays in its basic form.

Dugo means for a long time or long in the sense of duration.

It is an adverb, so it modifies the verb čekati (to wait):

• dugo čekati = to wait a long time

Because it is an adverb, it does not agree with nouns in gender, number, or case.

Compare:

• dug peron = a long platform → dug is an adjective
• dugo čekati = to wait a long time → dugo is an adverb

Because na here means on/at a location, and when it expresses location, it takes the locative case.

• peron = platform
• locative singular: peronu

So:

• na peronu = on the platform / at the platform

This is a location, not movement.

Compare:

• na peronu = on the platform → location
• na peron = onto the platform → movement

Because u can take different cases depending on whether it means location or movement.

Here the sentence describes movement:

• otići ćemo u čekaonicu = we will go into the waiting room

When u means movement into somewhere, it takes the accusative:

• čekaonica → accusative singular čekaonicu

Compare:

• u čekaonici = in the waiting room → location, so locative
• u čekaonicu = into the waiting room → movement, so accusative

Both word orders can occur, but otići ćemo is very common and natural.

This happens because ćemo is a clitic-like auxiliary, and Croatian often places clitics in the second position of their clause. Since the subordinate clause comes first:

• Ako budemo morali dugo čekati na peronu,
  then the new main clause begins:
• otići ćemo u čekaonicu

So in the main clause, the full verb otići comes first, and ćemo follows it.

You may also hear:

• Ako budemo morali dugo čekati na peronu, ćemo otići u čekaonicu

but that is not standard. The natural standard options are:

• otići ćemo
• sometimes mi ćemo otići, if the pronoun is expressed for emphasis

It is the future I tense.

It is made from:

• the infinitive otići = to go, to leave
• the auxiliary ćemo = we will

So:

• otići ćemo = we will go

This is the normal future tense used in the main result clause of an if sentence like this one.

Otići usually means to go away, to leave, or simply to go, depending on context.

In this sentence:

• otići ćemo u čekaonicu

it means something like:

• we’ll go to the waiting room
• we’ll head into the waiting room

The idea is that the speakers will leave the platform area and go somewhere else.

Because the sentence begins with a subordinate if-clause:

• Ako budemo morali dugo čekati na peronu = If we have to wait a long time on the platform

Then comes the main clause:

• otići ćemo u čekaonicu = we will go to the waiting room

Croatian normally separates this kind of introductory subordinate clause from the main clause with a comma.

It is understood, but not written.

Croatian often drops subject pronouns because the verb ending already tells you who the subject is:

• budemo = we
• ćemo = we will

So the sentence could be expanded as:

• Ako mi budemo morali dugo čekati na peronu, mi ćemo otići u čekaonicu.

But that sounds unnecessary unless you want emphasis or contrast. Normally Croatian just leaves mi out.

Yes. They are clearly related.

• čekati = to wait
• čekaonica = waiting room

This is useful vocabulary to notice, because Croatian often builds nouns from verbs in transparent ways.

So the sentence contains both:

• čekati = the action of waiting
• čekaonica = the place where people wait

A more literal breakdown would be:

• Ako = if
• budemo morali = we will have to / we end up having to
• dugo čekati = wait a long time
• na peronu = on the platform
• otići ćemo = we will go
• u čekaonicu = into the waiting room

So a very literal English version would be:

If we will have to wait a long time on the platform, we will go into the waiting room.

But natural English usually says:

If we have to wait a long time on the platform, we’ll go to the waiting room.

Yes, peron is a normal word, especially in travel contexts.

It means a platform, especially at a train or bus station.

So:

• na peronu = on the platform

For an English speaker, it helps to associate it with railway or station vocabulary.