Questions & Answers about 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:
- = I am not