I've seen a couple of sentences with "to be" + "infinitive with to" such as:
You are to do that.
I know it has to do with a kind of obligation but I'm still wondering what is the difference between it and using "have to".
e.g:
I am to tell you.
vs.
I have to tell you.
Best Answer
We were taught of these differences in modal verbs:
means an obligation due to higher order, like duty (of position or honour).
means that something is most prudent or sensible way of behaving.
means that was supposed to {infinitive} but didn't for some reason.
means obligation due to (possibly adverse) circumstances.
means obligation due to previous agreement or presumed way of behaving.
So, in your case,
means that according to some agreement or protocol I have the obligation/task to inform you.
means the circumstances force me to inform you (possibly against my wish, or I surrender to them).