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What is the negative of "must"? Is it "don't have to" or "mustn't"?


Sender Mr. Mansour A.A.
Position Teacher

The negative of must is don't have to.

When must means that something is obligatory, the negative is don’t have to because don’t have to means that there is no obligation. You can do something or not, as you wish. Mustn’t, on the other hand, means that you must NOT do something. In other words, the thing is forbidden.
 
You must stay home today. You have to take care of your little sister. (It is obligatory.)
You don’t have to stay home today. I'll be home to take care of your sister. (You have the option to go out.)
You mustn’t stay home today. They’re going to spray strong insecticide in the whole building. (It is obligatory that you NOT stay home. You must go out.)
 
Then again, when must is used for deduction, British English makes the negative with can’t. This means that something is logically impossible or that there is strong evidence to believe that it is not the case. This is the only form taught in the Hello! series.
 
Bernie must be home. All the lights are on.
Bernie can’t be home. All the lights are off.
 
 

ed. 22/03/12

 
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Sender  Mr. Ahmed Imam
Head teacher

I think that the negative of “must” is “don’t have to” or “doesn’t have to”, isn’t it? If so, please check your reply in question No. 20 in this group, The Prisoner of Zenda, page 13. The King said, “What would my brother Michael think if he saw us two together!” What is the right answer for this question? And why did Sapt say “I agree. He mustn’t go” not “I agree. He shouldn’t/doesn’t have to go.” Is it a strong reason?

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Editor’s reply:
 
I do not know where you are getting this question No. 20, so I cannot comment on it.
 
Must means that there is an obligation. Don’t have to means that there is no obligation. The person may choose to do something or to not do it.
 
As was noted in the answer above, must not means that there is an obligation to not do something. The action is forbidden. Sapt says that Rassendyll mustn’t go to the coronation because doing so is forbidden. He is not giving Rassendyll a choice to go or not to go.
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