Sutra moja sestra slavi rođendan u stanu.

In Serbian, the present tense is often used for a future action when a time word already makes the future clear, such as sutra = tomorrow.

So:

Sutra moja sestra slavi rođendan u stanu.

naturally means Tomorrow my sister is celebrating her birthday in the apartment/flat.

This is very common and natural in Serbian. You could also say:

Sutra će moja sestra slaviti rođendan u stanu.

That version uses the explicit future tense and is also correct.

Moja sestra is in the nominative case because it is the subject of the sentence — the person doing the action.

• sestra = sister
• moja = my

The adjective/pronoun moja agrees with sestra in:

• gender: feminine
• number: singular
• case: nominative

So moja sestra means my sister as the subject.

Because rođendan is the direct object of slavi — it is the thing being celebrated.

The verb slaviti takes the accusative case, so logically rođendan is accusative here.

However, rođendan is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: rođendan
• accusative: rođendan

That is why the form does not change.

Because u can be followed by different cases depending on the meaning:

• u + locative = in / inside a place, with no movement
• u + accusative = into a place, with movement toward it

Here, the meaning is location, not motion:

• u stanu = in the apartment / in the flat

Compare:

• Moja sestra slavi rođendan u stanu. = She is celebrating in the apartment.
• Moja sestra ulazi u stan. = She is going into the apartment.

So stanu is the locative singular of stan.

Stan usually means apartment or flat.

So u stanu most naturally means:

• in the apartment
• in the flat

It does not usually mean just room. A room is soba.

Depending on context, stan can also mean a living space or dwelling, but apartment/flat is the most common everyday meaning.

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

So Serbian often leaves that distinction to:

• context
• word order
• demonstratives such as taj, ovaj, onaj
• possessives such as moja

In this sentence:

• moja sestra clearly means my sister
• u stanu can be translated as in the apartment / in a flat, depending on context, though in the apartment is often the most natural English translation here

Yes. Serbian word order is fairly flexible, because case endings help show what each word is doing.

The sentence:

Sutra moja sestra slavi rođendan u stanu.

is perfectly natural, but you could also say:

• Moja sestra sutra slavi rođendan u stanu.
• Moja sestra slavi rođendan sutra u stanu.
• U stanu moja sestra sutra slavi rođendan.

These versions all have roughly the same basic meaning, but the focus changes:

• Sutra... emphasizes when
• Moja sestra... emphasizes who
• U stanu... emphasizes where

So the original sentence starts with sutra to foreground the time.

You can often omit moja if the context already makes it obvious whose sister you mean.

So both are possible:

• Sutra moja sestra slavi rođendan u stanu.
• Sutra sestra slavi rođendan u stanu.

But the version with moja is clearer and more natural if you are introducing the person or if you want to make the relationship explicit.

In English, we usually need my sister. In Serbian, possession is sometimes left understood, but using moja is completely normal.

Yes, absolutely.

Both are correct:

• Sutra moja sestra slavi rođendan u stanu.
• Sutra će moja sestra slaviti rođendan u stanu.

The difference is mainly stylistic:

• slavi = present tense used with future meaning, very natural with sutra
• će slaviti = explicit future tense

The shorter present-tense version is very common in everyday speech when the future time is already obvious.

In Serbian, slaviti rođendan is the normal way to say celebrate a birthday.

So:

• Moja sestra slavi rođendan = My sister is celebrating her birthday

If you want to say that it is simply someone’s birthday, without focusing on a celebration, Serbian often uses different wording, for example:

• Mojoj sestri je sutra rođendan. = It’s my sister’s birthday tomorrow.
• Sutra je mojoj sestri rođendan.

So:

• slavi rođendan = focuses on the celebration
• je rođendan = focuses on the fact that it is her birthday

A rough pronunciation guide:

• rođendan sounds approximately like rodjendan, but the đ is a soft sound, similar to the j sound in British English duke or the g in some pronunciations of education, though not exactly the same.
• sutra is roughly SOO-tra
• sestra is roughly SEH-stra
• stanu is roughly STAH-noo

A helpful breakdown:

• ro-đen-dan

The letter đ is softer than dž:

• đ = softer
• dž = like English j in jam

Learners often mix up đ and dž, so it is good to notice the difference early.