U pošti je veliki red, ali moja sestra kaže da će paket stići brzo.

Because the preposition u means in / at here, and when u shows location, the noun normally goes into the locative case.

• pošta = post office
• u pošti = in/at the post office

So:

• u poštu would usually mean into the post office (movement toward a place, accusative)
• u pošti means in/at the post office (location, locative)

This is a very common Serbian pattern:

• u školi = at school
• u gradu = in the city
• u kući = in the house

Here red means a line / queue, not order in the abstract sense.

So veliki red means a big line or a long queue.

The word red can have several meanings depending on context, for example:

• stajati u redu = to stand in line
• red i mir = order and peace
• prvi red = first row

In a sentence about the post office, red is naturally understood as a queue.

Serbian often uses je (is) in places where English might prefer there is.

So:

• U pošti je veliki red literally looks like In the post office is a big queue
• natural English: There is a long line at the post office

This is very normal in Serbian. You do not always need a direct equivalent of English there is.

Other similar examples:

• Na stolu je knjiga. = There is a book on the table.
• U sobi je tiho. = It is quiet in the room.

So je veliki red is a natural Serbian way to express the existence of a line in that place.

Because Serbian has no articles. It does not have direct equivalents of English a/an and the.

So veliki red could mean:

• a big line
• the big line
• simply a long queue

The exact meaning depends on context.

Likewise:

• moja sestra = my sister
• paket = a package / the package

Serbian speakers rely on context, word order, and sometimes demonstratives like taj (that) or ovaj (this) when they want to be more specific.

Moja sestra means my sister.

• moja = my
• sestra = sister

Yes, moja is the feminine singular form because sestra is a feminine noun.

Possessive words like moj / moja / moje agree with the noun in gender, number, and case:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

You do not always have to include the possessive if the context already makes it clear, but here moja sestra explicitly means my sister, so it is natural and useful.

In Serbian, da often introduces a subordinate clause, especially after verbs like say, think, know, want, and so on.

So:

• kaže = says
• da = that
• kaže da će paket stići brzo = she says that the package will arrive quickly

This is very common:

• Mislim da je tačno. = I think that it is correct.
• Znam da dolazi. = I know that he/she is coming.
• Kažu da će padati kiša. = They say that it will rain.

In everyday English, that is often omitted, but in Serbian da is usually kept.

Both are possible, but this version follows an important Serbian rule about clitics.

The word će is a clitic, a short unstressed word. Clitics usually go in the second position of their clause.

In the clause:

• da će paket stići brzo

the first element is da, so će comes right after it.

That is why da će paket stići brzo sounds natural.

You can also hear:

• Paket će stići brzo. = The package will arrive quickly.

Here paket is the first element, so će comes after it.

So the key idea is:

• in an independent clause: Paket će stići
• after da: da će paket stići

Stići means to arrive, to reach, or sometimes to make it in time, depending on context.

Here it means to arrive:

• paket će stići brzo = the package will arrive quickly

Stići is a perfective verb, which is very common for talking about a completed future event. The idea is that the package will successfully arrive at some point.

Compare:

• stizati = to be arriving / to arrive repeatedly / imperfective
• stići = to arrive (completed event) / perfective

For a one-time future result, stići is the natural choice.

Brzo is an adverb, meaning quickly / fast.

It modifies the verb stići:

• stići brzo = to arrive quickly

Putting brzo at the end is very natural in Serbian, especially in a simple statement like this.

You may also see adverbs in other positions for emphasis, but the end position is common and neutral:

• Paket će stići brzo.
• Paket će brzo stići.

Both are grammatical. The difference is mostly one of rhythm or emphasis, not basic meaning.

In this sentence, at the post office is usually the most natural English translation.

Literally, u pošti is in the post office, but English often says at the post office when talking about what is happening there in a general sense.

So:

• literal: In the post office, there is a big line
• natural English: There is a long line at the post office

This is a good example of how Serbian and English use place expressions a little differently.

Yes, Serbian word order is more flexible than English word order, but not completely free. You can move parts of the sentence for emphasis, topic, or style.

For example:

• U pošti je veliki red, ali moja sestra kaže da će paket stići brzo.
  Neutral and natural.

You could also hear:

• Veliki je red u pošti...
  This gives a slightly different emphasis.

• Moja sestra kaže da će brzo stići paket.
  Possible, but less neutral than the original.

Even though Serbian allows more movement, some positions sound more natural than others, and clitics like je and će still follow special placement rules. So word order can change, but not randomly.