This is the 113th most frequent Korean word.
않을
"않을" means "will not" or "not going to" (denotes negation in future or conditional situations).
The word '않을' is used here to express future negativity combined with the verb '믿다' (to believe).
나는 그것을 믿지 않을 것이다.
I will not believe it.
In this sentence, '않을' negates the action of speaking (하다).
그는 아무 말도 하지 않을 것 같아요.
It seems he will not say anything.
Here, '않을' is used to convey the negation of leaving (떠나다) combined with the noun '계획' (plan).