Proverite potpis; u protivnom zahtev nije važeći.

Proverite is the imperative form of proveriti (to check / verify), in 2nd person plural.

That means it can be used:

• for more than one person: check
• for one person politely/formally: please check

So in official instructions, Proverite potpis is a normal way to say Check the signature.

Also, the subject is not stated because Serbian usually omits subject pronouns when the verb ending already makes the person clear.

Because proveriti is perfective, and the sentence is telling you to perform one complete check.

• proveriti → proverite = check it / verify it
• proveravati → proveravajte would sound more like keep checking / check repeatedly / be in the habit of checking

In a short instruction on a form or notice, the perfective imperative proverite is the natural choice.

It is in the accusative, but with masculine inanimate singular nouns, the nominative and accusative often look the same.

So for potpis:

• nominative: potpis
• accusative: potpis

That is why you do not see any visible change, even though it is the direct object of proverite.

U protivnom means otherwise, if not, or more literally in the opposite case.

It is a common formal set phrase, especially in written instructions, warnings, and official language.

A fuller version is:

• u protivnom slučaju = otherwise / in the opposite case

So here, u protivnom introduces the consequence if the first instruction is not followed.

This is a slightly more advanced point.

U protivnom is really a shortened form of u protivnom slučaju.

In u protivnom slučaju:

• u takes the locative
• slučaju is locative singular of slučaj
• protivnom agrees with slučaju, so it is also locative singular

When slučaju is omitted, protivnom stays in that same form.

So the phrase is fixed as u protivnom.

Because Serbian does not have articles like English a/an/the.

So:

• potpis can mean a signature or the signature
• zahtev can mean a request/application or the request/application

The context tells you which meaning is intended. In this kind of official sentence, English usually translates them with the.

Because važeći is agreeing with zahtev.

• zahtev is masculine singular
• so the adjective is also masculine singular: važeći

Compare:

• zahtev nije važeći = masculine
• molba nije važeća = feminine
• odobrenje nije važeće = neuter

This is standard adjective agreement in Serbian.

Here važeći functions as an adjective meaning valid, effective, or in force.

It is related to the verb važiti:

• važiti = to be valid / to apply / to count
• važeći = valid / currently in force

Examples:

• važeći dokument = a valid document
• važeći zakon = a law currently in force
• zahtev nije važeći = the request is not valid

Yes. All of these are possible, but they differ a little in style.

• zahtev nije važeći = formal, neutral, administrative
• zahtev ne važi = more verbal, literally the request is not valid / does not apply
• zahtev je nevažeći = also possible, using the adjective nevažeći = invalid

The original sentence sounds especially natural in official written language.

The semicolon links two closely connected parts:

• Proverite potpis
• u protivnom zahtev nije važeći

It is stronger than a comma and is common in formal written Serbian, especially in official instructions and administrative texts.

So the punctuation helps give the sentence a neat, bureaucratic tone.

Yes, very formal.

It sounds like something you would read on:

• a form
• an official notice
• a legal or administrative document
• business or government paperwork

Two things contribute to that tone:

• proverite as a polite/formal imperative
• u protivnom as a formal linking phrase

A more everyday Serbian sentence would usually sound less stiff.

Because the sentence is written in Ekavian Serbian.

You may also see Ijekavian forms in other varieties:

• zahtev → Ekavian
• zahtjev → Ijekavian

The same kind of difference appears with the verb too:

• proverite → Ekavian
• provjerite → Ijekavian

So the version you were given is normal standard Serbian, just in the Ekavian pronunciation/spelling style.