Niko nije u kancelariji, jer je danas vikend.

This is a case of negative concord, which is normal in Serbian.

• niko = nobody / no one
• nije = is not

In Serbian, when you use a negative word like niko, the verb is also usually negative:

• Niko nije u kancelariji.

That is the correct natural Serbian pattern.
So even though English avoids double negatives in standard grammar, Serbian requires them in sentences like this.

You will see the same thing with other words too:

• Nikada ne radim vikendom. = I never work on weekends.
• Ništa ne znam. = I know nothing.

So the short answer is: because Serbian grammar works differently from English here.

Nije is the 3rd person singular present negative form of the verb biti (to be).

So:

• je = is
• nije = is not

In this sentence:

• Niko nije u kancelariji = No one is in the office.

It is made from the verb biti, but in modern Serbian you should just learn nije as the standard negative form.

Some useful related forms:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

Because after u meaning in, Serbian uses the locative case for location.

• dictionary form: kancelarija = office
• locative singular: kancelariji

So:

• u kancelariji = in the office

This is a very common pattern:

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

Important note: u does not always take the same case.

• u + locative = location, in/at
• u + accusative = motion into, into

Compare:

• On je u kancelariji. = He is in the office.
• On ide u kancelariju. = He is going into/to the office.

Because je is a clitic, and Serbian clitics usually go in second position in a clause.

In the clause:

• jer je danas vikend

the word jer starts the clause, and the clitic je comes right after it.

This is very normal Serbian word order.

So although English thinks more in terms of because today is the weekend, Serbian prefers:

• jer je danas vikend

not usually:

• jer danas je vikend (this sounds unnatural in neutral speech)

This second-position behavior is important for short forms like:

• je
• sam
• si
• se
• ga
• mu
• etc.

Because the sentence has two separate clauses, and each clause needs its own verb.

1. Niko nije u kancelariji

   • verb: nije = is not

2. jer je danas vikend

   • verb: je = is

So the full structure is:

• No one is in the office, because it is the weekend today.

In Serbian, when you say something like today is the weekend, you still use the verb to be:

• Danas je vikend.

You cannot just say:

• Danas vikend
  in a normal complete sentence.

Because Serbian has no articles. It does not have words equivalent to English a/an and the.

So:

• kancelarija can mean an office or the office
• vikend can mean a weekend or the weekend

The exact meaning depends on context.

In this sentence:

• u kancelariji is naturally understood as in the office
• danas je vikend is naturally understood as today is the weekend / it’s the weekend today

Serbian speakers rely on context rather than articles.

Jer means because and introduces the reason:

• Niko nije u kancelariji, jer je danas vikend.

It connects the main statement with its explanation.

You can sometimes use other expressions for because, for example:

• zato što
• pošto (in some contexts)

Examples:

• Niko nije u kancelariji, zato što je danas vikend.
• Niko nije u kancelariji, pošto je danas vikend.

These are similar in meaning, though jer is one of the most direct and common ways to say because.

It can often mean either, depending on context.

Literally, u kancelariji is in the office, but in natural English it may also be translated as at the office if that sounds better.

So:

• Niko nije u kancelariji
  can be understood as
• No one is in the office or
• No one is at the office

The Serbian phrase focuses on being at that location, and English may choose whichever version sounds more natural in the situation.

Yes, Serbian word order is fairly flexible, but not every version sounds equally neutral.

The given sentence:

• Niko nije u kancelariji, jer je danas vikend.

is a very natural neutral version.

You may also hear:

• Jer je danas vikend, niko nije u kancelariji.

This puts the reason first:

• Because it’s the weekend today, no one is in the office.

However, you must still respect clitic placement, so je stays near the beginning of its clause.

That is why:

• jer je danas vikend = natural
• jer danas je vikend = not the usual neutral order

Vikend is a very common Serbian noun meaning weekend, borrowed from English long ago and fully integrated into the language.

It behaves like a normal Serbian masculine noun:

• vikend = weekend
• vikenda = of the weekend
• vikendom = on weekends / during the weekend

For example:

• Danas je vikend. = Today it’s the weekend.
• Ne radim vikendom. = I don’t work on weekends.

So even though it comes from English historically, in Serbian it is just an ordinary everyday word.

A careful pronunciation would be roughly:

• Niko nije u kancelariji, jer je danas vikend.
• NEE-koh NEE-yeh oo kan-tseh-lah-REE-yee, yer yeh DAH-nahs VEE-kend

A few helpful pronunciation notes:

• j is pronounced like English y
  • je sounds like ye
• nj is like the ny in canyon
• č / ć / dž / đ are different sounds, but this sentence does not contain them
• c in kancelariji sounds like ts
• r is rolled or tapped more than in English

You do not need perfect pitch accent as a beginner, but it helps to keep the rhythm smooth and not stress every word equally.