Kada ulaziš u zgradu, skreni levo; moj stan je tamo.

Because Serbian changes the case after u depending on whether you mean movement into something or location inside something.

• u zgradu = into the building → accusative
• u zgradi = in the building → locative

So in Kada ulaziš u zgradu, the idea is motion: you are going into the building.

A useful contrast:

• Ulaziš u zgradu. = You are entering the building.
• Ti si u zgradi. = You are in the building.

Ulaziš is the 2nd person singular present tense of ulaziti / ulaziti-type verb ulaziti/ulaziti? More standardly, the verb is ulaziti = to be entering / to enter repeatedly / habitually, while the perfective partner is ući = to enter.

Here, ulaziš matches you in an informal singular sense: you enter / you are entering.

The ending -iš tells you it is:

• present tense
• 2nd person singular
• used with implied ti = you

So:

• ja ulazim = I enter / I am entering
• ti ulaziš = you enter / you are entering
• on/ona ulazi = he/she enters

Serbian often uses the present tense in time clauses introduced by kada when talking about something that happens in the future or as part of an instruction.

So Kada ulaziš u zgradu literally looks like When you are entering the building, but in context it works like:

• When you enter the building...
• As you go into the building...

This is very normal in Serbian. English often prefers a present form after when too, so the structure is not actually that far from English.

Yes, absolutely. That is a very common alternative.

Compare:

• Kada ulaziš u zgradu...
• Kad uđeš u zgradu...

The difference is mainly about aspect and style:

• ulaziš comes from an imperfective verb, which can sound a bit more like as you’re entering or refer to the process/situation
• uđeš comes from the perfective verb ući, and often sounds more like once you enter / when you go in

Also:

• kada is a bit more neutral/full
• kad is a shorter everyday variant

In real speech, Kad uđeš u zgradu, skreni levo is probably the most natural everyday version.

Skreni is the imperative form, meaning turn.

It comes from the verb skrenuti = to turn (usually a perfective verb in this sense). The speaker is giving a direct instruction to one person:

• skreni levo = turn left

This is the singular informal imperative.

Compare:

• skreni = turn! (to one person, informal)
• skrenite = turn! (to more than one person, or to one person formally)

It is informal singular, because it uses forms for ti:

• ulaziš
• skreni

These are what you say to:

• one friend
• one family member
• one person you are addressing informally

If you wanted the formal or plural version, you would say:

• Kada ulazite u zgradu, skrenite levo; moj stan je tamo.

That can mean either:

• you all
• you (formal singular)

Levo means left here, used as an adverb: to the left / leftward.

After verbs of turning or movement, Serbian often uses this adverb directly:

• skreni levo = turn left
• idi desno = go right

You do not need an article like in English. Serbian has no the/a articles.

Also note a regional/standard variation:

• levo = common in Serbian, especially Ekavian
• lijevo = Ijekavian form

Both are correct depending on the variety.

Usually, in this kind of instruction, levo by itself is the normal choice:

• Skreni levo. = Turn left.

Using na levo is generally not the standard neutral way here. For basic direction commands, just use the adverb:

• levo
• desno
• pravo

Examples:

• Idi pravo. = Go straight.
• Skreni desno. = Turn right.

Stan means apartment or flat.

So:

• moj stan = my apartment / my flat

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

Examples:

• Moj stan je mali. = My apartment is small.
• Moja soba je mala. = My room is small.

Because stan is a masculine singular noun, and the possessive adjective must agree with it.

So:

• moj stan = my apartment
• moja kuća = my house
• moje selo = my village

Agreement in Serbian depends on:

• gender
• number
• case

Since stan is masculine singular in the nominative here, the correct form is moj.

Tamo means there.

So moj stan je tamo literally means:

• my apartment is there

It points to a location away from the speaker or previously indicated in context.

A useful mini-contrast:

• ovde / ovdje = here
• tu = there / here nearby / there at that spot
• tamo = there, over there

In this sentence, tamo makes sense because the speaker is directing someone to a place after turning left.

Yes. Serbian word order is fairly flexible.

These are all possible, with slightly different emphasis:

• Moj stan je tamo. = neutral
• Tamo je moj stan. = emphasis on there
• Moj je stan tamo. = possible, with emphasis on my

The version in your sentence is a simple, neutral way to say it.

Because Kada ulaziš u zgradu is a subordinate time clause: When you enter the building.

In Serbian, when that clause comes first, it is normally followed by a comma before the main clause:

• Kada ulaziš u zgradu, skreni levo.

That comma works much like in English:

• When you enter the building, turn left.

The semicolon links two closely related statements:

• skreni levo
• moj stan je tamo

It is stronger than a comma, but not as separate as a full stop. In English and Serbian writing, it can be used when the second clause explains or adds to the first.

You could also write this naturally as:

• Kada ulaziš u zgradu, skreni levo. Moj stan je tamo.
• Kada ulaziš u zgradu, skreni levo, moj stan je tamo. — less standard, because it risks being a comma splice
• Kada ulaziš u zgradu, skreni levo: moj stan je tamo. — possible if the second part is seen as an explanation

So the semicolon is a stylistic punctuation choice that cleanly joins the two related ideas.

You would change the verb forms from ti to vi forms:

• Kada ulazite u zgradu, skrenite levo; moj stan je tamo.

Changes:

• ulaziš → ulazite
• skreni → skrenite

This can mean either:

• you all enter... turn left
• you (formal singular) enter... turn left

They mean the same thing here: when.

• kada = full form, slightly more formal or careful
• kad = shorter, very common in everyday speech

So both are fine:

• Kada ulaziš u zgradu, skreni levo.
• Kad ulaziš u zgradu, skreni levo.

In casual conversation, kad is extremely common.