Obligation and Necessity

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Modals for Obligation and Necessity
Main Rules for Obligation and Necessity

Must and Mustn’t
Use MUST to command someone from a position of authority. Could be a judge or a parent, etc.
  • You must pay the fine within 30 days.
  • You must tidy your room if you want to go out later.
Use MUSTN'T to prohibit or warn someone, possibly indirectly (-ve) reporting the rules of authority.
  • You mustn't drive without a licence.
  • You mustn't do that again!
  • You mustn't talk to strangers.
Use MUST and MUSTN'T to talk about an obligation you feel personally.
  • I must finish this today!
  • They must eat their vegetables too!
  • We mustn't be late for the train.
Have to, Don’t have to and Needn’t
Use HAVE TO in a similar way to MUST, to give obligations from authority - here though it may be indirectly (+ve) reporting the requirement.
  • They have to do what the teacher says.
  • You have to drive on the left in Ireland.
  • I'm sorry, you have to be 18 to see this film.
Use DON'T HAVE TO and NEEDN'T to express a LACK of necessity (NOT a negative order).
  • I don't have to do this until tomorrow.
  • On Saturdays they don't have to wake up early.
  • She needn't come if she doesn't want to.
"Don't have to/Needn't" are VERY different than "Mustn't"
  • You mustn't drink that it's poisonous. (Don't drink it!)
  • You needn't drink the milk if you don't want to. (You decide to drink or not)
  • You don't have to drink alcohol, you know? (You decide to drink or not)
Form of Must
  • Has no future form. Use WILL HAVE TO.
    • Tomorrow I will have to work all day.
  • Has no past form. Use HAD TO.
    • I had to pay the fine immediately.
  • MUST is not followed by 'TO'
    • I must go.
    • NOT I must to go.
Form of Needn't
  • NEEDN'T is not followed by 'TO'
    • I needn't go.
    • NOT I needn't to go.
[Sourced: EnglishAdam.com]

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