Stigla je pogrešna porudžbina.

Because it agrees with porudžbina, which is a feminine singular noun.

In Serbian past tense, the main verb is formed with:

• the auxiliary je
• plus a past participle that agrees in gender and number with the subject

So:

• stigao = masculine singular
• stigla = feminine singular
• stiglo = neuter singular
• stigli / stigle = plural

Since porudžbina is feminine, you get stigla je porudžbina.

The dictionary form is stići, which means to arrive.

In this sentence, stigla is the feminine singular past participle of stići.

So the pattern is:

• stići = to arrive
• stigla je = arrived / has arrived (for a feminine singular subject)

Je is the 3rd person singular present form of biti (to be) and it is used as an auxiliary to form the past tense.

So Serbian past tense here is:

• stigla je = literally something like has arrived
• in natural English, this is often translated simply as arrived

This is the normal Serbian past tense structure:

• On je stigao. = He arrived.
• Ona je stigla. = She arrived.
• Porudžbina je stigla. = The order arrived.

Because pogrešna must agree with porudžbina in:

• gender
• number
• case

Since porudžbina is feminine singular nominative, the adjective also has to be feminine singular nominative:

• pogrešan = masculine
• pogrešna = feminine
• pogrešno = neuter

So:

• pogrešan broj = wrong number
• pogrešna porudžbina = wrong order
• pogrešno mesto = wrong place

It is in the nominative singular.

Here, porudžbina is the subject of the sentence, and subjects are normally in the nominative case.

So in:

• Stigla je pogrešna porudžbina.

the thing that arrived is pogrešna porudžbina, so it is nominative.

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

All of these are possible:

• Stigla je pogrešna porudžbina.
• Pogrešna porudžbina je stigla.

They mean basically the same thing, but the focus is slightly different.

• Stigla je pogrešna porudžbina.
  This can sound more natural when introducing the event: The wrong order arrived.
• Pogrešna porudžbina je stigla.
  This puts more focus on the wrong order itself.

Also, Serbian often places the auxiliary je in the second position of the clause, which helps shape word order.

Yes. Serbian uses its standard past tense here:

• stigla je

English may translate this as:

• The wrong order arrived or
• The wrong order has arrived

Depending on context, Serbian perfect often covers both of those English ideas.

So grammatically it is past tense in Serbian, even if the most natural English translation may be simple past.

It can mean either, depending on context.

Serbian has no articles like the or a, so the sentence itself does not explicitly mark that distinction.

So:

• pogrešna porudžbina can be the wrong order
• or a wrong order

In real use, context usually makes it clear. In many situations, English would most naturally translate it as the wrong order.

In standard Serbian, not here.

You normally need the auxiliary in the past tense:

• Stigla je pogrešna porudžbina.

Leaving it out would sound nonstandard or incomplete in an ordinary declarative sentence.

So for learners, the safe rule is:

• don’t omit the auxiliary in this kind of past tense sentence.

Stići is a perfective verb.

That matters because perfective verbs usually describe a completed event. Arrival is naturally a completed action: the order arrived.

So stići is a very normal choice here.

Its imperfective partner is often stizati, meaning something like to be arriving / to arrive repeatedly depending on context.

Compare:

• Porudžbina je stigla. = The order arrived.
• Porudžbina je stizala kasno. = The order used to arrive late / was arriving late.

No. A very common alternative is narudžbina.

Both can mean order, especially in shopping or delivery contexts. Which one sounds more natural can depend on region, company style, or personal preference.

So you may also see:

• Stigla je pogrešna narudžbina.

That means the same thing.

A rough pronunciation guide:

• po-rudž-bi-na

Key sounds:

• ž sounds like the s in measure
• dž sounds like the j in jam

So:

• ž = voiced zh
• dž = j sound

Also:

• pogrešna has š, which sounds like sh
• Stigla is roughly STEE-gla

In normal usage, it usually means the wrong order was delivered/arrived.

In other words, the package or order that came was not the one expected.

So the adjective pogrešna modifies porudžbina directly:

• wrong order

It does not mean that the order made a mistake by itself; it means the order was incorrect or not the intended one.

Yes, it sounds natural and normal.

It is the kind of sentence you might use when:

• a delivery arrives
• a customer service issue comes up
• someone opens a package and sees it is not what they ordered

Very natural alternatives include:

• Stigla je pogrešna porudžbina.
• Pogrešna porudžbina je stigla.
• Dobili smo pogrešnu porudžbinu. = We got the wrong order.

The last one is also extremely common in real conversation.