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.


I have to tell you.


We were taught of these differences in modal verbs:

Must {infinitive}

means an obligation due to higher order, like duty (of position or honour).

Should {infinitive}

means that something is most prudent or sensible way of behaving.

Ought to {infinitive}

means that was supposed to {infinitive} but didn't for some reason.

Have to {infinitive}

means obligation due to (possibly adverse) circumstances.

Be to {infinitive}

means obligation due to previous agreement or presumed way of behaving.

So, in your case,

I am to tell you.

means that according to some agreement or protocol I have the obligation/task to inform you.

I have to tell you.

means the circumstances force me to inform you (possibly against my wish, or I surrender to them).


Obligation is usually expressed with "must+bare infinitive". As the modal verb must can only be used in present tense, for the other tenses a substitution ( have to do) is used. "I have to do it" is a shortening of "I have the obligation to do it".

"You are to come to the boss" is a shortening of "You are ordered to come to the boss". In some cases you can think of "obliged/demanded and similar forms" instead of "ordered".


I am to is more formal than I am supposed to. Both of these have a more neutral tone (i.e. less emotive) than I have to.

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