Dok smo čekali na kolodvoru, otišli smo u čekaonicu jer je svako slobodno sjedalo na peronu bilo mokro.

Dok means while, so it introduces an action that was in progress in the past. Croatian normally uses the perfect with an imperfective verb for this kind of background action:

• čekati = to wait, be waiting
• smo čekali = we were waiting / we waited

So Dok smo čekali... means While we were waiting...

The form čekasmo belongs to the old/simple past system (aorist/imperfect area), which is not the normal neutral choice in modern everyday Croatian. A learner should usually use smo čekali.

This is mainly about aspect.

• čekati is imperfective: it describes an ongoing process, repeated action, or background situation.
• otići is perfective: it describes a completed action, a single event with an endpoint.

So in the sentence:

• Dok smo čekali... = while we were waiting
• otišli smo u čekaonicu = we went into the waiting room

This is a very common contrast in Croatian:

• ongoing background action = imperfective
• main completed event = perfective

English often does something similar with was waiting vs went.

This is because smo is a clitic in Croatian. Clitics usually go in the second position of their clause, not necessarily after the verb.

So:

• Dok smo čekali...
  Here the first element in the clause is Dok, so the clitic smo comes right after it.

• otišli smo u čekaonicu
  Here the first element is otišli, so smo comes after it.

This can feel strange to English speakers, but it is very normal in Croatian. Clitics like sam, si, je, smo, ste, su and short pronouns often follow this second-position rule.

This is a case-and-preposition issue.

na kolodvoru

• na
  • locative here means at/on a place
• kolodvoru is the locative singular of kolodvor
• So: na kolodvoru = at the station

u čekaonicu

• u
  • accusative here shows motion toward a place
• čekaonicu is the accusative singular of čekaonica
• So: u čekaonicu = into the waiting room

A useful rule:

• where? → often locative
• to where? → often accusative

Compare:

• u čekaonici = in the waiting room
• u čekaonicu = into the waiting room

Croatian uses certain prepositions naturally with certain places, and peron normally goes with na.

• na peronu = on the platform

Even though English says on the platform, learners sometimes expect u because a platform is also a kind of place. But Croatian idiomatically uses na here.

This is something you often just have to learn by usage:

• na kolodvoru
• na peronu
• u čekaonici

svako slobodno sjedalo means every free seat or every available seat.

All three words are in the neuter singular nominative because they agree with each other:

• svako = every (neuter singular)
• slobodno = free/available (neuter singular)
• sjedalo = seat (neuter singular noun)

The noun sjedalo is neuter, so the words describing it also have to be neuter.

Compare:

• svaki slobodni stolac = every free chair/seat masculine
• svaka slobodna klupa = every free bench feminine
• svako slobodno sjedalo = every free seat neuter

Because the subject is singular, not plural.

The subject is:

• svako slobodno sjedalo = every free seat

Even though English every refers to many items in meaning, grammatically it is treated as singular. Croatian does the same here.

So the verb and predicate stay singular neuter:

• svako sjedalo je bilo mokro

Not:

• su bila mokra — that would require a plural subject such as sva slobodna sjedala

Compare:

• Svako sjedalo je bilo mokro. = Every seat was wet.
• Sva sjedala su bila mokra. = All the seats were wet.

Because mokro agrees with sjedalo, which is neuter singular.

• mokra = feminine singular
• mokar = masculine singular
• mokro = neuter singular
• mokra/mokre/mokri can also appear in plural depending on gender and style

Since sjedalo is neuter singular:

• sjedalo je bilo mokro

This is the normal predicate-adjective pattern:

• stol je bio mokar = the table was wet
• klupa je bila mokra = the bench was wet
• sjedalo je bilo mokro = the seat was wet

Yes, slobodan/slobodna/slobodno literally means free, but in this context it means unoccupied or available.

So slobodno sjedalo is:

• a free seat
• an available seat
• an empty seat

This is very common in Croatian, just like in English:

• Je li ovo mjesto slobodno? = Is this seat free?
• Nema slobodnih stolova. = There are no free tables.

So here svako slobodno sjedalo means every available seat, not every liberated seat or anything like that.

Croatian usually does not need subject pronouns when the verb form already shows the person.

For example:

• smo čekali already tells you the subject is we
• otišli smo also tells you we

So adding mi would usually only be done for:

• emphasis
• contrast
• clarification

For example:

• Mi smo čekali, a oni su otišli. = We were waiting, and they left.

In your sentence, no pronoun is needed because the verb forms are clear.

Yes, jer and zato što can both mean because.

So these are both possible:

• ...jer je svako slobodno sjedalo na peronu bilo mokro.
• ...zato što je svako slobodno sjedalo na peronu bilo mokro.

Difference

• jer is short, common, and very natural
• zato što is also very common, sometimes a bit fuller or more emphatic

In many everyday sentences, either works. Here jer is a simple, natural choice.

Both relate to transport, but they are not always the same.

kolodvor

Usually means a larger station/terminal, especially:

• train station
• bus station

stanica

Often means a stop or station in a broader sense:

• bus stop
• tram stop
• police station
• station in general

So na kolodvoru suggests a proper station building/area, which fits well with:

• peron = platform
• čekaonica = waiting room

That makes the whole sentence sound like a railway or bus-station setting.

The noun čekaonica is a feminine noun ending in -a.

For many feminine nouns of this type:

• nominative singular: čekaonica
• accusative singular: čekaonicu

Since u here expresses movement into the place, Croatian uses the accusative:

• u čekaonicu

Compare:

• Čekaonica je velika. = The waiting room is big.
  nominative
• Otišli smo u čekaonicu. = We went into the waiting room.
  accusative
• Bili smo u čekaonici. = We were in the waiting room.
  locative

Possibly, but the meaning would shift a little.

sjedalo

Usually means a seat as a seating place/surface, often in transport contexts:

• train seat
• bus seat
• seat on a platform bench, depending on context

stolac

Usually means a chair

sjedište

Can mean seat, but also headquarters, and it often sounds less natural for a simple seat in some everyday contexts unless the situation specifically calls for it.

So sjedalo is a good choice here because it fits the idea of available seating at a station/platform.

Because dok emphasizes simultaneity: one action was happening while another happened.

• Dok smo čekali... = While we were waiting...

If you used kad, the meaning would shift more toward when:

• Kad smo čekali... is possible in some contexts, but it does not fit this exact ongoing-background sense as neatly.

So:

• dok = while, during the time that
• kad = when

Here the sentence is setting the scene: waiting was already in progress, and during that time they went into the waiting room. That is why dok is a very natural choice.

Yes, Croatian word order is more flexible than English, although some versions sound more natural than others.

For example, you could also say:

• Otišli smo u čekaonicu dok smo čekali na kolodvoru, jer je svako slobodno sjedalo na peronu bilo mokro.

But the original is better because it presents the background first, then the action, then the reason:

1. Dok smo čekali na kolodvoru — background
2. otišli smo u čekaonicu — main action
3. jer... — reason

So the original order is very natural and easy to follow, even though Croatian does allow some variation.