have to

I found a lot of examples with explanations on using this words applying to people, but not to inanimate objects. In the case of inanimate objects, such explanations as “obligatory”, “moral correctness”, “advise” etc. does not work.
What the verbs should be used in the next sentences?

The car _______ turn when the driver turns the steering wheel.
The car _______ not start moving spontaneously.
The (computer) program _______ solve the equation.
If an error occures, the program _______ indicate a message.

I think, different variants are possible and they have different meanings.

  • 1
    To take one example, 'The car must turn when the driver turns the steering wheel.' If this follows 'We've carried out the service very thoroughly.', the usage is that of epistemic modality: 'There's no way [as far as I'm concerned] that the car won't turn!' But if the preceding sentence was 'You've got to sort this out!', the modality shown in 'The car must turn when the driver turns the steering wheel.' is deontic. A ... Commented Sep 26, 2017 at 10:47
  • fuller definition from Wikipedia is: 'The sentence containing the deontic modal generally indicates [here, points to, in the preceding sentence] some action that would change the world so that it becomes closer to the standard or ideal.' Consider this as a paraphrase of 'You must make sure that the car turns when ...'. Commented Sep 26, 2017 at 10:47
  • -1 For the parade to attract the largest crowd, the sun (___) be out. Insert any of your four words for a good English sentence. Commented Sep 26, 2017 at 11:39
  • The verb should works in all your sentences. Commented Sep 26, 2017 at 12:03
  • @Peter Shor, what about the other verbs?
    – Diusha
    Commented Sep 27, 2017 at 7:10

1 Answer 1


Must and should are English modal auxiliary verbs. The other English modal auxiliary verbs are may, might, shall, will, would, can, and could.

Ought (to) and have to are periphrastic modal auxiliary idioms. Some others include be likely (to), be able to, be required (to), be possible (that), and be expected.

All modals, whether periphrastic or not, have two kinds of meaning: Deontic and Epistemic.

Deontic modality has to do with obligations, permissions, and affordances conceived in a stratified social network:

  • She may go to the dance with you, but she has to be back by midnight.

Deontic modality doesn't apply to things, outside metaphors. Things need Epistemic modality,
which has to do with possibility and necessity, from logical and practical/mechanical causes:

  • The ball may pass through either gate, but it has to go in this hole in order to score.

Epistemic must and have to mean 'necessary', but deontically they mean 'obligated to'.

Epistemic should and ought to mean 'likely, probable'; deontically they mean 'morally obliged to'.

Epistemic can, could, may, and might refer to possibilities; deontically they refer to permissions.

  • 1
    +1 Another useful post. While I'm here and we're talking modality, are there any modal words/phrases used as determiners? Commented Oct 5, 2017 at 19:41
  • @Araucaria-Nothereanymore. Not really, but in a way you can think of modals, negatives, and quantifiers as being Operators (functors with Superpowers like Binding and Polarity) and quantifiers are the ones that act as determiners. Commented Jan 6, 2020 at 21:44

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