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You are to do your homework without watching any TV.
The students are not to travel by subway without a friend.

Can't we also write these sentences like this:

You are going to do your homework without watching any TV.
The students are not going to travel by subway without a friend.

Is there a difference? I am not talking about just little difference like "less formal". And are there sentences where "be to do" can be used and "be going to do" cannot be used?

Is there an original similarity? Has "Going" fallen away over time? Or are these two completely different expressions?

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  • On the other hand "be" in the first sense can indicate an arrangement or future state, rather than an order or obligation: Merriam-Webster (aux verb 4) gives "I am to interview him today." "She was to become famous." So there's a strong overlap. But "going" is more common than the simple "be", so it's definitely not fallen away over time. It's kind of a big question though, and describing all the variations isn't trivial.
    – Stuart F
    Commented Jul 23 at 14:18
  • @EdwinAshworth How 'bout finding the dupe instead of answering in comments? :(
    – tchrist
    Commented Jul 23 at 14:23
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    Good luck searching for "be": this has a very short answer, this question is related, this was closed
    – Stuart F
    Commented Jul 23 at 14:26
  • This is closer: What's quasi-modal be? But it doesn't cover the difference asked about.
    – Stuart F
    Commented Jul 23 at 14:28
  • To be to do something can have two senses. As you say, it can mean to be going to or planning to do something. But in the examples you give, it means to be required to do something - so the first one means "You must do your homework without watching any TV".
    – Kate Bunting
    Commented Jul 23 at 17:19

1 Answer 1

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  • The students are not going to travel by subway without a friend

may be jussive (in the sense of laying down a rule) ... or merely declarative (in the sense of voicing an assumed certainty, a future event).

But

  • The students are not to travel by subway without a friend

can only be jussive.

Some examples are ambiguous, while others seem restricted to a single interpretation. Thus

  • I am to interview him today [thanks to Stuart F]

indicates an obligation, which could be a loose arrangement or a contractual necessity. But

  • She was to become famous

can realistically have no hint of obligation, just of a timeline.

And then again,

  • I was to interview him the following day

may be speaking either of an arrangement (and we're not sure whether the interview took place), or merely of an event we know took place the following day (and which might not even have been pre-arranged).

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    A common kink in this question is that it's a common rhetorical device to express jussive intent as a forceful declaration: "You are gonna eat your dinner right now, young man!"
    – Andy Bonner
    Commented Jul 23 at 20:33

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