I am to you are to he, she, it is to we are to you are to they are to I was to you were to he, she, it was to ... The forms of "to be" followed by an infinitive are used to express obligation. This construction is just one more shade of meaning in the same class as "must", "have to", "should", "ought to", "be supposed to", "be expected to", the last two mentioned being the closest in meaning (to my ear, at least) to this particular construction.
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