This is the 11th most frequent Serbian word.
не
"Не" in Serbian means "no" or "not" in English.
Here, 'не' negates the verb 'знам', indicating that the action (knowing) is not happening.
Не знам како да ти помогнем.
I don't know how to help you.
'Не' is used within the clause 'није истина' to negate the truth of the statement.
Мислила сам да то није истина.
I thought that it wasn't true.
In this sentence, 'не' is used as part of the negation construction 'није' to indicate that the book does not belong to the speaker.