Hi,
Welcome to the Forum.
Would you tell me what is the difference between
1) he is gone The verb here is 'is'. 'Gone' is used here as an adjective. It's like saying 'he is tall' or 'he is finished'.
he has gone. 'has gone' is a verb form, the present perfect. The sentence means that sometime in the past, he went, so now he's not here.
2) I have to go It's necessary for me to go. It may be because I want to, or for some reason outside me.
I am to go It's necessary for me to go. The reason is from outside me, probably because someone else is telling me it is necessary.
Best wishes, Clive