Gospodin čeka u policijskoj postaji.

Because u means location here, so it requires the locative case.

The basic form is policijska postaja. In the locative singular, that becomes policijskoj postaji. Both words change because the adjective policijska must agree with the noun postaja in gender, number, and case.

A useful contrast:

• u policijskoj postaji = in / at the police station
• u policijsku postaju = into the police station

So:

• location → locative
• movement into → accusative

Croatian u often covers meanings that English expresses with both in and at.

With buildings and institutions, Croatian commonly says u even when natural English may prefer at. So u policijskoj postaji is a normal Croatian way to say something that English might translate as either in the police station or at the police station, depending on context.

Because a police station is treated as an enclosed place or institution, and Croatian normally uses u for that.

Very roughly:

• u = in, inside, at certain places/institutions
• na = on, onto, and also at some places by idiom

So you say:

• u školi
• u bolnici
• u policijskoj postaji

You cannot reliably translate English at as either u or na word-for-word. You usually have to learn which preposition each place takes.

It can mean different but related things depending on context.

• gospodin on its own often means gentleman or a polite/formal man
• before a surname, it means Mr., as in gospodin Horvat

In this sentence, used alone, it sounds polite and somewhat formal. It is not just any neutral man; it has more of a gentleman / sir / respectable man feel than muškarac or čovjek.

Because Croatian has no articles.

So gospodin could mean:

• a gentleman
• the gentleman
• sometimes simply the man

And policijska postaja could be:

• a police station
• the police station

English has to choose a or the, but Croatian usually leaves that to context.

Because gospodin is in the nominative, which is the usual case for the subject of the sentence.

Also, u policijskoj postaji is a prepositional phrase in the locative, so it cannot be the subject here. That leaves gospodin as the person doing the action, and čeka agrees with it in third person singular.

Čeka is the 3rd person singular present tense of čekati.

It can mean both:

• waits
• is waiting

Croatian present tense often covers both the English simple present and present continuous. In this sentence, the most natural English translation is usually is waiting, but the Croatian form itself is just the regular present tense.

Also, čekati is imperfective, so it is the normal verb for an ongoing action like waiting.

Because Croatian does not form the present continuous the way English does.

English says:

• is waiting

Croatian simply uses the present tense:

• čeka

So one Croatian form can correspond to either waits or is waiting in English, depending on context.

Because Croatian čekati normally takes a direct object without a preposition.

English says:

• wait for someone
• wait for the bus

Croatian says:

• čekati nekoga
• čekati autobus

So if you add the thing being waited for, it usually goes in the accusative, with no word equivalent to English for.

Examples:

• Gospodin čeka policajca. = The gentleman is waiting for the police officer.
• Gospodin čeka autobus. = The gentleman is waiting for the bus.

In your sentence, the object is simply omitted, so čeka just means is waiting.

Yes. Croatian word order is fairly flexible because case endings carry a lot of grammatical information.

This sentence has a neutral order:

• Gospodin čeka u policijskoj postaji.

But other orders are possible, for example:

• U policijskoj postaji gospodin čeka.
• Čeka gospodin u policijskoj postaji.

These alternatives change the focus or emphasis, not the basic meaning. English is usually much less flexible here.

You may hear both in the wider South Slavic area, but policijska postaja is a standard and very natural Croatian expression.

Policijska stanica is also understandable and is common in some neighboring standards/varieties, but if you are learning standard Croatian as used in Croatia, policijska postaja is the safer choice.

So for this sentence, u policijskoj postaji is perfectly normal Croatian.