Nikako ne birajte uplatu unapred.

Here nikako means by no means, absolutely not, or under no circumstances. It makes the warning much stronger than a simple ne birajte.

So:

• Ne birajte uplatu unapred. = Don’t choose advance payment.
• Nikako ne birajte uplatu unapred. = Definitely do not choose advance payment.

A useful contrast is:

• nekako = somehow
• nikako = in no way / absolutely not

Those two are easy to mix up.

Yes, nikako is negative in meaning, but Serbian still normally keeps the regular verbal negation ne.

So the structure is:

• nikako
  • ne
    • verb

That is standard Serbian. In English, you might feel this is like a double negative, but in Serbian it is normal.

Examples:

• Nikako ne idi tamo. = Do not go there under any circumstances.
• Nikako ne zovite taj broj. = By no means call that number.

Birajte is the imperative, second person plural/formal, of the verb birati (to choose).

So:

• biraj = choose! — singular informal
• birajte = choose! — plural or polite singular

In this sentence, because it is negative:

• ne birajte = don’t choose

So the whole sentence is a negative command or warning.

Yes. In Serbian, the plural form is also used as a polite/formal singular.

So birajte can mean:

• speaking to more than one person
• speaking politely to one person

If you were talking to one friend informally, you would say:

• Nikako ne biraj uplatu unapred.

If you were addressing several people, or one person politely:

• Nikako ne birajte uplatu unapred.

This is mostly about aspect.

• birati = imperfective
• izabrati = perfective

In Serbian, negative commands often prefer the imperfective:

• Ne birajte...
• Ne uzimajte...
• Ne otvarajte...

That sounds natural and standard.

A form like Ne izaberite... would sound wrong. If you really wanted to use the perfective idea, Serbian would normally use nemojte:

• Nemojte izabrati uplatu unapred.

But in ordinary instructions and warnings, ne birajte is the most natural choice.

Because uplatu is in the accusative case, which is used for the direct object of the verb.

The dictionary form is:

• uplata = payment

But after birati (to choose), you need the thing being chosen in the accusative:

• birati šta? = to choose what?
• uplatu

So:

• birajte uplatu = choose payment
• ne birajte uplatu unapred = don’t choose payment in advance

This is a normal feminine noun pattern:

• nominative: uplata
• accusative: uplatu

Uplata unapred is a set phrase meaning payment in advance or advance payment.

It is made of:

• uplata = payment
• unapred = in advance / beforehand

So literally it is something like payment beforehand.

In the sentence, because uplata is the object of the verb, it becomes:

• uplatu unapred

Yes, unapred is written as one word. It is an adverb, meaning in advance, beforehand.

Examples:

• Platite unapred. = Pay in advance.
• Sve smo dogovorili unapred. = We arranged everything beforehand.

It does not change for case, gender, or number.

You may also see the Ijekavian spelling unaprijed in Bosnian, Croatian, or some Serbian usage. In this sentence, unapred is the standard Ekavian form.

Sometimes yes, but the nuance is different.

• nikad = never
• nikako = by no means / absolutely not / in no way

So:

• Nikad ne birajte uplatu unapred. = Never choose advance payment.
• Nikako ne birajte uplatu unapred. = By no means choose advance payment.

In many practical situations, the translation into English may look similar, but nikako sounds more like a strong prohibition or warning, not just a statement about time.

The given word order is very natural:

• Nikako ne birajte uplatu unapred.

Serbian word order is fairly flexible, but different orders change the emphasis.

For example:

• Uplatu unapred nikako ne birajte.
  This emphasizes advance payment.

The original version is the most neutral and idiomatic for a warning. Putting nikako before the negated verb is especially natural.

It sounds like a strong warning or firm instruction.

Because it uses:

• the imperative birajte
• negation ne
• the intensifier nikako

it is stronger than a mild suggestion.

Compare:

• Bolje je da ne izaberete uplatu unapred. = It’s better not to choose advance payment. — softer
• Ne birajte uplatu unapred. = Don’t choose advance payment. — direct
• Nikako ne birajte uplatu unapred. = Absolutely do not choose advance payment. — strongest of the three

So the tone is clear and emphatic.