Hai = Indicates singular presence , example: Woo acha aadmi hai. (He is a good fellow )
Hain = Indicates more than one presence, example: Woo achche loog hain ( They are good people)
Hoon= Indicates presence , example: Mae acha aadmi hoon. ( I am a good fellow)
Hey = Specially indicated, example: Woo hey acha aadmi hai. ( He is the nice fellow)