Večeras ćemo slaviti sa gostima u stanu.

Večeras is an adverb meaning tonight / this evening. In many contexts, English can translate it either way.

A useful thing to notice is that Serbian often uses večeras by itself, without a preposition. English says tonight, not in tonight, and Serbian works similarly here.

This is the future tense in Serbian.

It is formed with:

• a short form of hteti = to want / will
• plus the infinitive

So here:

• ćemo = we will
• slaviti = to celebrate

Together:

• ćemo slaviti = we will celebrate

The full set of short future forms is:

• ću = I will
• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you (plural) will
• će = they will

Because ćemo is a clitic. In Serbian, clitics usually go in second position in the sentence.

So in:

• Večeras ćemo slaviti sa gostima u stanu.

the first element is Večeras, and the clitic ćemo comes right after it.

This is very normal Serbian word order.

Yes. That is also correct.

Serbian future can often appear in two common patterns:

• ćemo slaviti
• slavićemo

So both are possible:

• Večeras ćemo slaviti sa gostima u stanu.
• Slavićemo večeras sa gostima u stanu.

They mean the same thing. The choice is often about style, rhythm, or emphasis.

After s / sa when it means with, Serbian uses the instrumental case.

So:

• gost = guest
• gosti = guests
• gostima = with guests / guests in the instrumental plural

That is why the sentence has:

• sa gostima = with guests

Both mean with.

Sa is basically a longer variant of s, often used when pronunciation is easier or more natural that way. In everyday Serbian, speakers may use sa quite often.

So:

• s gostima
• sa gostima

Both can be heard, and sa gostima sounds completely natural.

Because u can take different cases depending on the meaning:

• u + locative = location, in
• u + accusative = direction, into

Here the meaning is location:

• u stanu = in the apartment

So stanu is the locative singular of stan.

Use u stan when there is movement into the apartment.

Compare:

• Slavimo u stanu. = We are celebrating in the apartment.
• Idemo u stan. = We are going into the apartment / to the apartment.

So:

• u stanu = where something happens
• u stan = where someone is going

Stan means apartment / flat.

It does not usually mean room.
For room, Serbian normally uses soba.

So:

• u stanu = in the apartment / in the flat
• u sobi = in the room

Because Serbian has no articles.

English distinguishes:

• a guest
• the guest
• the apartment
• an apartment

Serbian usually does not. The exact meaning depends on context.

So:

• sa gostima can mean with guests or with the guests
• u stanu can mean in an apartment or in the apartment

The situation tells you which one is meant.

Serbian word order is fairly flexible, but it is not random. Different orders can change emphasis.

For example:

• Večeras ćemo slaviti sa gostima u stanu.
  Focus starts with tonight.

• Sa gostima ćemo večeras slaviti u stanu.
  Stronger emphasis on with guests.

• U stanu ćemo večeras slaviti sa gostima.
  Stronger emphasis on in the apartment.

The original sentence is neutral and natural.

These two sounds are similar, and English speakers often mix them up.

• č is a harder sound, roughly like ch in church
• ć is softer; many learners hear it as something between ty and a soft ch

So:

• Večeras starts with ve-che-...
• ćemo starts with a softer sound than English ch

You do not need to make it perfect immediately, but it is good to notice that Serbian treats č and ć as different letters.

Yes, slaviti can stand on its own, as in this sentence:

• Večeras ćemo slaviti. = We will celebrate tonight.

But it can also take an object, for example:

• slaviti rođendan = to celebrate a birthday
• slaviti pobedu = to celebrate a victory

So in your sentence, the idea is simply we will celebrate, without saying exactly what is being celebrated.