The word “not” is considered as an adverb because it is used to modify adjectives, verbs, and other adverbs. For instance, in the sample sentence below:
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.