I was reading a book and found a really strange sentence:

"She is to be back by nightfall".

Does it mean either she has got to be back by nightfall or she is supposed to be back by nightfall, or does it mean something else?

  • 3
    Or simply that that she will be back by nightfall. It could mean any of those things. It's not possible to tell without more context. May 28, 2020 at 14:14
  • Sounds like it's setting a rule.
    – Justin
    May 28, 2020 at 14:49
  • 2
    The meaning is normally closer to she is supposed to be back (that's what she's expected to do, and/or she has been instructed to do). The meaning of she has got to be back by nightfall is normally closer to she must be back (there is no choice, it's inevitable that she will be back). In some contexts, that "required" outcome might be equivalent to a "law of nature" that cannot possibly be broken under any circumstances - much stronger than simply what someone ought to do (or what's expected to happen). May 28, 2020 at 15:54
  • the verb to be + time restraint, usually means: supposed to be
    – Lambie
    Nov 4, 2020 at 16:27

2 Answers 2


Without more context, we can’t be certain, but I think these are the two most likely candidates:

  • She is (expected) to be back by nightfall.
  • She is (required) to be back by nightfall.

Which is meant should be clear from context.


Yes, this is laying down a rule: she has to be back by nightfall. Since it is spoken in the third person, I would expect that it was told to a third party who has the duty of ensuring she is back by nightfall.

Also, it's an odd and even creepy way of phrasing it.

  • 1
    That is only one interpretation. She boarded a train. According to its schedule, she is to be back by nightfall. That doesn't have anything to do with a rule or an order. It's a simple statement of fact (an expectation or prediction) rather than anything to do with obligation. May 28, 2020 at 16:35
  • That would be a technically grammatical but even odder structure for the meaning.
    – Mary
    May 28, 2020 at 16:58
  • I've heard it used frequently enough in that context that it doesn't sound particularly odd to me. May 28, 2020 at 16:59

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