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Is there any difference between have to”, “must”, and “should”?

If there is some difference between them, when do I have to use (nor not use) each of the constructions below?

  • have to do something
  • must do something
  • should do something
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Yes, there are differences.
These three modals refer to deontic obligation of various varieties.
Like all modals, they're used in many idioms, all irregular.
And they have different syntax, too.

There have been many dissertations written about modals and necessity/obligation,
so I won't belabor the point here. Well, I'll try not to, anyway.

Must and should are modal auxiliary verbs, and contrast in their deontic sense in the strength (and often in the source) of the obligation.
Must is stronger; it's used in orders.
Should is weaker; it's used in advice.

Plus, quite often, must obligations come from outside (fate, weather, boss, parents, mullah -- somebody or something who or which can enforce the order), while should obligations come from your own conscience, or from people who seem to care about you.

Have to is a modal paraphrase (also called a periphrastic modal, which is "paraphrase" in Greek), of must. (Ought to is a modal paraphrase of should. I won't deal with it here.)

Modal paraphrases come in handy in English when we need a past tense or a negative in the sentence, because modal auxiliary verbs have very limited syntax -- they require all negatives to follow them, and they are not inflected for tense.

  • He has to leave now. = He must leave now.
    Normally, in the present and in the affirmative, they're identical.
    But with a negative, there are two possibilities
  • He doesn't have to leave. ≠ He must not leave.
    The first one means he has no obligation to leave,
    but the second means he has an obligation not to leave; i.e, he must stay.
    And with a past obligation, the auxiliary verb just won't stretch.
  • He had to leave at 4pm. ≠ *He musted leave at 4pm.
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  • I would only add that most people are unfamiliar with the term deontic, meaning “Of or relating to duty, obligation, etc.” Hm, I wonder whether there’s any literature on people being motivated to use things like “be able to” and “have to” so that they can carry tense markers in ways that true modals seldom can.
    – tchrist ♦
    Commented Aug 25, 2013 at 18:31
  • Deontic vs epistemic (two kinds of modal meaning) is explained in the link. Every modal expression can be interpreted either way, though there are lots of contexts that force one or the other interpretation. As epistemic modals, must means 'necessary, in my opinion' (He must be home by now), while should means 'likely, in my opinion' (He should be home by now). Have to still means must epistemically, but it's not as common outside deontic usages.
    – John Lawler
    Commented Aug 25, 2013 at 18:45
  • By the way, the Deontic-Epistemic opposition was mentioned in a recent Lexicon Valley podcast.
    – John Lawler
    Commented Jul 21, 2020 at 19:17
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Examples

  • I have to tell you this - someone/something is making me tell you

  • I must tell you this - it is important so I will.

  • I should tell you this - if I didn't I would feel bad. I might not actually tell you but I should

Here are more differences

http://www.onestopenglish.com/support/ask-the-experts/grammar-questions/grammar-teaching-the-modals-ought-to-should-must-and-have-to/146365.article

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  • So Should have not "Important" mode?
    – Amirreza Nasiri
    Commented Aug 25, 2013 at 16:48
  • It can be important, but I might not actually do it
    – mplungjan
    Commented Aug 25, 2013 at 16:49

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