Negation: нисам, немам, не знам

In Serbian не (not) is normally a separate word before the verb: не знам (I don't know), не разумем (I don't understand), Она не пије кафу.
Two verbs are different: the negation fuses with them into one word.

• бити → нисам, ниси, није, нисмо, нисте, нису: Нисам гладан (I'm not hungry).
• имати → немам, немаш, нема, немамо, немате, немају: Немају деце (They have no children). The thing you lack is usually in the genitive.

Never say не сам or не имам. Unlike сам, нисам is stressed, so it can open a sentence.
The past tense is built with сам, си, је…, so its negative uses the same fused forms: Нисмо знали (we didn't know), Није звала (she didn't call). Double negatives are normal: Нисте ништа рекли.