Birajte poštansku dostavu; u protivnom idite u policijsku stanicu.

Because birajte and idite are the plural/formal imperative forms.

In Serbian, these forms are used:

• for more than one person, or
• for one person in a polite/formal way.

So this sentence sounds like an instruction addressed to:

• the general public,
• a customer,
• or multiple people.

Compare:

• Biraj poštansku dostavu. = Choose postal delivery. (singular, informal)
• Birajte poštansku dostavu. = Choose postal delivery. (plural or polite/formal)

The same applies to:

• Idi = go! (informal singular)
• Idite = go! (plural or polite/formal)

Birajte is the imperative of the verb birati = to choose / to select.

Here the sentence is giving an instruction, so Serbian uses the imperative:

• birati → biraj / birajte

This verb is imperfective, so birajte can sound like a general instruction such as choose / select in menus, forms, or official directions.

A learner may also notice another possible verb:

• izabrati = to choose (perfective)
• imperative: izaberite

Very roughly:

• birajte = choose/select (process or general instruction)
• izaberite = choose/pick (more like making a single completed choice)

In many practical contexts, both can appear, but birajte is very natural in interface-style or procedural instructions.

Because birati takes a direct object, and the direct object is usually in the accusative case.

The basic noun phrase is:

• poštanska dostava = postal delivery (nominative)

But after birajte:

• birajte poštansku dostavu = choose postal delivery (accusative)

Both words change because the adjective must agree with the noun:

• poštanska → poštansku
• dostava → dostavu

This is a very common pattern in Serbian:

• Vidim dobru knjigu. = I see a good book.
• Kupite novu kartu. = Buy a new ticket.
• Birajte poštansku dostavu. = Choose postal delivery.

It is in the accusative singular.

Why:

• dostava is a feminine singular noun
• it is the direct object of birajte

So:

• nominative: poštanska dostava
• accusative: poštansku dostavu

For many feminine nouns ending in -a, the accusative singular changes to -u:

• stanica → stanicu
• dostava → dostavu

And the adjective changes to match:

• poštanska → poštansku
• policijska → policijsku

U protivnom means otherwise or if not.

It is a fixed expression, short for something like:

• u protivnom slučaju = in the opposite case / otherwise

So in this sentence:

• Birajte poštansku dostavu; u protivnom idite u policijsku stanicu. means:
• Choose postal delivery; otherwise, go to the police station.

This phrase is common in formal instructions and official writing.

Because Serbian often uses u protivnom as a shortened fixed phrase.

The fuller version is:

• u protivnom slučaju = otherwise / in the opposite case

But in real usage, especially in formal or written language, slučaju is often omitted:

• u protivnom = otherwise

So both are possible, but u protivnom is concise and natural.

Because after u, Serbian uses different cases depending on whether there is:

• movement toward a place, or
• location in a place.

Here we have movement:

• idite u policijsku stanicu = go to the police station

So Serbian uses the accusative:

• policijsku stanicu

Compare:

• u policijsku stanicu = into/to the police station (motion, accusative)
• u policijskoj stanici = in the police station (location, locative)

This is a very important pattern:

• Idem u školu. = I’m going to school.
• Ja sam u školi. = I am at school.

It is also accusative singular feminine.

The base form is:

• policijska stanica = police station (nominative)

After u with motion:

• u policijsku stanicu = to the police station (accusative)

Again, the adjective and noun must agree:

• policijska → policijsku
• stanica → stanicu

So both feminine noun phrases in this sentence are in the accusative:

• poštansku dostavu
• policijsku stanicu

They are very close, but not always identical in style.

• poštanska dostava = postal delivery / mail delivery
• dostava poštom = delivery by mail / by post

The first is a more compact noun phrase with an adjective:

• poštanska modifies dostava

The second uses an instrumental phrase:

• poštom = by mail/post

In many contexts they can translate similarly, but poštanska dostava sounds like the name of a delivery option or service, which fits this sentence very well.

The semicolon links two closely connected instructions while keeping them clearly separate.

The logic is:

• Choose postal delivery;
• otherwise, go to the police station.

A semicolon works well because the second part depends on the first part logically, but it is still a full instruction on its own.

You could also see similar punctuation with a comma or a full stop in some contexts, but the semicolon gives the sentence a formal, written, instruction-like tone.

Not naturally in this formal context.

• u protivnom = otherwise (formal, official, instructional)
• inače usually means otherwise / generally / as for that / normally, depending on context

In official instructions, u protivnom is much more appropriate.

For example:

• Platite na vreme; u protivnom sledi kazna. = Pay on time; otherwise a penalty follows.

Using inače here would usually sound less precise or less formal.

It sounds correct and natural, but definitely formal/official.

Clues:

• Birajte and idite in polite/plural imperative
• u protivnom as a formal connector
• the procedural, instruction-like structure

This is the kind of Serbian you might see in:

• official instructions
• application procedures
• government or administrative notices
• form or website guidance

In everyday spoken Serbian, someone might phrase it more casually, but for formal instructions this is perfectly normal.