Referentni broj je na potvrdi o predatom prtljagu.

Broj is in the nominative singular because it is the subject of the sentence.

• referentni broj = reference number
• je = is

So the basic structure is:

• Referentni broj = the subject
• je = the verb to be
• na potvrdi o predatom prtljagu = where it is

This is a very standard Serbian sentence pattern: X je na Y = X is on Y.

Referentni broj means reference number.

• broj = number
• referentni = reference / referring / identifying

In practical contexts like travel, tickets, baggage, forms, or customer service, referentni broj is the number used to identify something in the system.

Because na here expresses location (on), and when na means location, it takes the locative case.

The noun is:

• potvrda = confirmation, receipt, slip

Its locative singular is:

• potvrdi

So:

• na potvrdi = on the receipt / on the confirmation slip

This is a very common pattern:

• na stolu = on the table
• na karti = on the map
• na potvrdi = on the receipt/slip

Because Serbian usually uses na for something written on a surface or document.

So:

• na potvrdi = on the receipt/slip/document

This is similar to:

• na papiru = on the paper
• na karti = on the map
• na računu = on the bill/invoice

Using u would suggest inside something, not simply written on it. For a document or ticket stub, na is the natural choice.

Here potvrda means something like a receipt, confirmation slip, or stub.

In the phrase:

• potvrda o predatom prtljagu

it refers to the document you get after handing over or checking in baggage.

Depending on context, English might translate it as:

• baggage receipt
• baggage check
• baggage claim receipt
• confirmation of checked baggage

Here o means about or regarding.

So:

• potvrda o predatom prtljagu = receipt/confirmation about the handed-over baggage

In Serbian, o takes the locative case, which is why both words after it are in locative forms.

Compare:

• priča o gradu = a story about the city
• informacija o letu = information about the flight
• potvrda o prtljagu = confirmation about the baggage

Because after o, Serbian uses the locative case, and the adjective must agree with the noun.

The base noun is:

• prtljag = baggage, luggage

Its locative singular is:

• prtljagu

The word predatom comes from predat / predati and here means something like:

• handed over
• submitted
• checked in

Since it describes prtljag, it must match it in case, number, and gender:

• predatom prtljagu = about the handed-over / checked baggage

Because prtljag follows the preposition o, and o requires the locative case.

So:

• nominative: prtljag
• locative: prtljagu

This is a normal masculine noun pattern in Serbian:

• grad → o gradu
• kofer → o koferu
• prtljag → o prtljagu

Yes. Literally, predat means handed over or submitted.

In travel language, that works naturally for baggage you have given to the airline or check-in desk. In English, we usually say:

• checked baggage
• checked-in baggage

But Serbian may express that idea with predat prtljag or prtljag koji je predat.

So the Serbian is natural, even if the most literal English wording sounds slightly different.

Yes. Serbian word order is more flexible than English word order.

This sentence:

• Referentni broj je na potvrdi o predatom prtljagu.

could also be rearranged, for example:

• Na potvrdi o predatom prtljagu je referentni broj.

Both are grammatically possible, but they do not feel exactly the same in emphasis.

• Referentni broj je... puts focus first on the reference number
• Na potvrdi... puts focus first on the location/document

The original version is very neutral and natural.

Yes. Referentni is an adjective modifying broj.

Since broj is:

• masculine
• singular
• nominative

the adjective must match it:

• referentni broj

This is standard adjective agreement in Serbian. For example:

• veliki kofer = big suitcase
• tačan broj = exact number
• referentni broj = reference number

A natural reading would usually stress the key information word, depending on context.

For example:

• Referentni broj je na potvrdi o predatom prtljagu.

If someone is asking Where is the reference number?, the stress may fall on:

• potvrdi

If the important information is which number, then:

• referentni broj

Serbian stress is not marked in normal writing, so learners usually just learn the sentence as a whole and pick up natural emphasis from context.