Am To and Have to
To express the idea of one's necessity:
Incorrect : You are to go there.
Correct : You have to go there.
To express the idea of external pressure:
Incorrect : I have to attend the class.
Correct : I am to attend the class.
Note: "You have to go there' implies that you fully appreciate the necessity of going there. Nobody is putting any pressure on you. But 'I am to attend the class' implies that whether I like the class or not, I must attend it because someone is compelling me to do it.
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