In the English language, the word “not” is solely categorized as an Adverb.
They have been warned not to enter the room.
The word “not” is used to modify the verb “to enter.” Because it modifies a verb, it is then categorized as an adverb.
a. used with an auxiliary verb or “be” to form the negative
- She would not say.
b. used as a short substitute for a negative clause
- Maybe we’ll have a test, but I hope not.
c. used to express the negative of other words
- Not all old cars are considered classics.