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Obligation/Need - Modals - Func. Grammar
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Use of modals – Obligation and need: (don’t) have to, must, be allowed to, should, need (all of the uses of MUST are here, too).
by michelle
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Expressing (strong/external) obligation, lack of obligation/need: “(not) have to” (not “must”!!!)

  • I have to go to work (obligation)
    I have to getup at 8.00 (obligation)
    You have to hand in a composition (obligation)
  • I have to go now – I have some duty that makes me go now (obligation)
  • You don’t have to do it, if you don’t want to (no need/obligation =You needn’t* do it).
  • Do we have to staytill 8? (At work) (obligation)
    Do we have to wait for them? (obligation)

“Need” or “have to”?
If you just want to indicate a need, it’s better you use “need”, but both are very close, very similar.

  • I have to go to the toilet (dramatic!) – I need to go to the toilet (more neutral)
  • I have to go to the dentist’s (true, very dramatic!)
  • I need to go to the dentist’s (what a courageous and responsible person!)
  • I need to makesome copies (you are aware of your needs) – I have to makesome copies

In the neg., the auxiliary “need” means “no hacer falta”, but we prefer “not have to”:

  • You needn’t do this exercise (old-fashioned!) = You don’t have to do this exercise.

What does “must” express?
“Must” is not generally used to indicate obligation. You think so, because you learned it with traffic signs: “You must stop here,” “You mustn’t park here,” but - wow! - saying that to someone is a bit too much! When people communicate orally they tend to avoid “You must”. A police person, for instance, is likely to avoid “You must(n’t)…” and likely to tell you: “You CAN’T park here, madam/sir” or, if they want to be more explicit about the legal obligation, “You ARE NOT ALLOWED TO park here”. If that police person tells you “You must(n’t)…,” then he/she is angry or concerned about a risk you have been running, or feels it is appropriate to QUOTE THE RULE. “Must” is so often avoided to indicate obligation, that when the authorities issue leaflets to inform people about their obligations, they use SHOULD, or CAN. Yes, “should” is the modal we use for ADVICE, to give advice, but it’s also got the sense of strong advice, this is, we use it when we want to INFORM ABOUT AN OBLIGATION IN A POLITE WAY.

  • You should fasten your seatbelt, madam/sir.
    You should pay your taxes before March 1.
  • Your carry-on bag cannot exceed (weight or size).

In notes and when rules need to be reported because people might behave contrary to the rule, we can use “You must” but remember these contexts are not those where we develop personal relationships. At high school, for instance, you may find this notice: “You must not leave the premises during school hours”. In exams you may hear “You must turn off/switch offyour cellphones/mobile phones” (but we actually prefer, “Cellphones/Mobile phones must be turned off/switched off,” precisely because we feel “You must” is too much!). In/On a plane, you are likely to hear (and read) “You must fastenyour seatbelts”, “You must extinguish your cigarettes” (put out), “Cellphones must be switched off,” “Your carry-on bags must fitunder the seat or in the overhead locker/compartment.”

Mostly, we use “must”…

bullet to indicate we feel morally bad about something we are not doing and should be doing! (with first persons – I/We) – not for obligation, only for moral obligation!

  • I must go now – I feel guilty about not leaving. I feel my own moral pressure. It’s so dramatic!

bullet to indicate you’re really angry!

  • You must stop making that noise! You’re driving me crazy!
  • You mustn’t cross when the lights are red!!! (You have just had a fright and thought your loved one was going to be run over by a car)

bullet to warn/alert of a danger (this is related to the former point

  • You must fasten your seatbelt (if you don’t, your security could be at risk)
  • You mustn’t smoke in this area (because there are inflammable objects)

bullet to recommend things we love, to recommend emphatically (fanatically! J) Notice it’s a very different use. It’s got nothing to do with obligation, danger or being angry.

  • You MUST watch it! (the movie I saw yesterday)
  • You MUST read this story – it’s amazing!

bullet for deduction.

  • Where is María? // She must be downstairs (present)
  • She must have missed the bus (past)
  • There must be a mistake here (present)
  • There must have been a misunderstanding. I didn’t ask for steak, I asked for some soup.