Which sentence is correct?
You are a hysterical... or You are an hysterical...and why....what is the rule.
It depends on how you spell the word that comes after an a or an an.
If it begins with a spoken vowel, you need the an.
I come in an hour. -> hour=[our] - spoken vowel
A universe without life is not a universe. -> universe=[you-niverse] - no spoken vowel
You are a hysterical person. -> hysterical=[hysterical]
(I doubt it's [isterical].
This is an honor. -> honor=[onor]
I have found an ultimate weapon. -> ultimate=[altimate]
So as said, I think it's "a hysterical".
P.S. Not a teacher or native speaker.