Snabdevač je proverio obračun.

This is the normal Serbian past tense, called perfekat. It is built with:

• a present-tense form of biti = to be
• a past participle

So here:

• je = is/has as an auxiliary
• proverio = past participle of proveriti = to check / verify

Together, je proverio means checked or has checked, depending on context.

Because the past participle agrees with the subject in gender and number.

Here the subject is snabdevač, which is:

• masculine
• singular

So the correct form is proverio.

Compare:

• Snabdevač je proverio obračun. = masculine singular
• Snabdevačica je proverila obračun. = feminine singular
• Preduzeće je proverilo obračun. = neuter singular

The dictionary form is proveriti.

So:

• proveriti = to check, to verify
• proverio = checked / has checked, masculine singular

When you look up verbs in Serbian, you look them up in the infinitive, so here that would be proveriti.

Snabdevač is in the nominative singular because it is the subject of the sentence, the person doing the action.

So:

• snabdevač = supplier
• it answers the question who checked?
• therefore it is nominative

Obračun is in the accusative singular because it is the direct object, the thing that was checked.

It looks the same as the basic form because obračun is a masculine inanimate noun, and for many masculine inanimate nouns:

• nominative singular = accusative singular

So:

• nominative: obračun
• accusative: obračun

That is completely normal in Serbian.

In Serbian, the object is often identified by case, not only by word order. In this sentence, the object is obračun, and its role is clear from the sentence structure and the verb.

Also, because obračun is masculine inanimate, its accusative form happens to be identical to its nominative form. So even though the ending does not change, it is still functioning as the direct object.

It can mean either one.

The Serbian past tense used here often covers both:

• English simple past = checked
• English present perfect = has checked

Which translation sounds best depends on context.

So Snabdevač je proverio obračun could be:

• The supplier checked the statement/calculation
• The supplier has checked the statement/calculation

Proveriti is perfective. That means it presents the action as completed.

So je proverio suggests that the checking was finished.

If you wanted an imperfective idea such as was checking, used to check, or kept checking, you would use an imperfective verb, for example proveravati:

• Snabdevač je proveravao obračun.

That has a different feel from je proverio, which points to a completed check.

Yes, Serbian word order is fairly flexible, but the neutral order here is very natural:

• Snabdevač je proverio obračun.

A very important detail is that je is a clitic, so it usually goes in second position in the clause.

For example:

• Danas je snabdevač proverio obračun.
• Obračun je proverio snabdevač.

These are possible, but they change emphasis or focus. The original sentence is the most neutral and straightforward.

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

So snabdevač can mean:

• a supplier
• the supplier

And obračun can mean:

• a calculation / a statement
• the calculation / the statement

Context tells you which meaning is intended.

It depends on context. Obračun can mean things like:

• calculation
• settlement
• statement
• account calculation
• payroll calculation

So in a business or accounting context, it often refers to some kind of formal financial calculation or statement, not just a casual math calculation.

A rough pronunciation guide is:

• Snabdevač ≈ snab-de-vach
• je ≈ ye
• proverio ≈ pro-ve-ri-o
• obračun ≈ ob-ra-chun

A few useful sound notes:

• j sounds like English y in yes
• č sounds like ch
• Serbian vowels are usually pronounced clearly and separately, so proverio has four clear vowel sounds

So the whole sentence is roughly:

snab-de-vach ye pro-ve-ri-o ob-ra-chun