Here's why:
* "Am" is the present tense form of the verb "to be."
* "Not" is an adverb that negates the verb.
* Past participles are used to form perfect tenses (e.g., present perfect, past perfect) and passive voice. They typically end in "-ed" or "-en."
To express a negative past tense, you would use "was not" or "wasn't."
For example:
* Present: I am not ready.
* Past: I was not ready.
* Past perfect: I had not been ready.