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Suppose my friend invites me to play football. But I have planned to study. So, what of the followings should be my reply:

  1. I have to study. (Since, I am not obliged by someone else, should I use it? Or, should I use it because I think I have made a firm plan and I am obliged by it)

  2. I am to study. (I find it stilted, since it doesn't have connotation of firmness. It just indicates a plan).

3 Answers 3

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"Be to", oddly enough, means that you have been directed or destined to do something by someone else.

I can't. My mother says I am to clean my room.

I am to go to London in a fortnight and report to the major.

Whereas there can be almost any source of obligation in "have to":

I have to study if I want to pass.

I have to use the washroom.

As you will note, this means "be to" is more or less a subset of "have to". "Have to" would work for the first two examples (losing some flavour of Britain and archaism, by the way), but "be to" could not work for the second two examples.

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  • Where does "must" stand on this scale?
    – IS4
    Commented Dec 26, 2017 at 2:02
  • @IS4 Good question. It's closer to "have to" but does suggest a bit of outside influence or necessity. Hence, "I must go to London" means "I have to go to London in order to accomplish something or because someone needs me" or similar — not because someone told you to, but also not because of an internal inclination. (Unless you mean it as sort of an implied obligation, e.g. "I simply must see Paris before I die!") Hence "I must go to the washroom" is a little odd, as though it weren't your own inclination to go. Commented Jun 18, 2022 at 17:39
  • But it's worth noting that almost nobody uses "must" in that sense anyway, at least not without affectation. Rather, it's usually limited to the sense of drawing a conclusion: "There's no one at the door... It must have been the wind." Or "He must've returned that book to the library already. I can't find it anywhere." Commented Jun 18, 2022 at 17:41
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"have to" sort of implies 'enforcement/necessity/obligation' and "are to" sort of implies 'obligation/supposingness'.

  • I have to do the dishes. (It is necessary even if I don't want to; I'm enforced to do it)
  • I am to do the dishes. (I'm obliged since nobody else wants to; I'm supposed to do it)

Mostly "to be to" isn't used in spoken English, only in written (mostly formal) English. Unlike have to which is very close to should, "to be to" is much closer to must in degree.

If you should study and you cannot not do it then "have to" or "should" is your choice, but if you must study but it isn't so strict then better use "must" and don't overuse "are to".


This is what I have been taught

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I would suggest "need to". It indicates a necessity, without implying an externally imposed obligation. But "have to" is also acceptable.

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