What’s the Difference? Must Not vs. Don’t Have To

Must Not EmoticonThere is a big difference between telling someone “You mustn’t drink alcohol” and telling him “You don’t have to drink alcohol.”

Meaning:

Must not means that the action is illegal or dangerous.  If I overhear the first speaker, I think this is a doctor telling someone that his health will be ruined if alcohol crosses his lips. He might have a serious heart condition or be taking medication that even a drop of alcohol will impact negatively.  Perhaps the speaker is a flight or driving school instructor warning students against operating an aircraft or a car while under the influence of alcohol.  The message is that the action could kill you and everyone else.  At the very least, you will lose your license.

Do not have to, in contrast, suggests that something isn’t necessary, that it’s a choice.  The second speaker is saying that there are many different types of beverages in the world—water, fruit juice, soda, tea, coffee—alcohol is only one among them.  If you don’t want to drink alcohol, for reasons of taste, health or religion, it isn’t necessary.  You can decide.

Knowing the difference between the two modals, you can see why it would be strange to say: “You don’t have to fall asleep while you are driving.”  It’s not just a choice.  If you fall asleep, you and others might die.  What you mean is “You mustn’t fall asleep while you are driving.”

Grammar:

Both must not and do not have to are modal verbs.  They are followed by a verb in the simple or base form (no changes to its structure).

Must not is much stronger than do not have to.

Also, must not can only be used to talk about the present or the future.  To express this idea in the past, I would have to use “was forbidden to” or “wasn’t allowed to.”  In contrast, do not have to can be used in the past.  For example, “When I was young, I didn’t have to wear a uniform to school.  I could choose what I wore.  Now, I don’t have to wear a uniform either because my job doesn’t require one.”

Remember to change do to does when your subject is he / she / it:  “My friend doesn’t have to dress up for work.”

Practice:  Read the sentences below and decide whether it is warning you away from an action or offering you a choice.  You can check your answers below.

warning 2                             choice

WARNING                       OR                          CHOICE

  1. You ______________________________ wear formal clothes when you are not working.
  1. You ______________________________ hurt a child.
  1. You ______________________________ lie on your resume.
  1. You ______________________________ carry a weapon into an airport or airplane.
  1. You ______________________________ get married.
  1. You ______________________________ carry cash.
  1. You ______________________________ use an electric device in the bath tub.
  1. You ______________________________ buy Apple products.
  1. You ______________________________ be careless about your passwords.
  1. You _______________________________ have children.

 

Answers: 

  1. don’t have to
  2. mustn’t
  3. mustn’t
  4. mustn’t
  5. don’t have to
  6. don’t have to
  7. mustn’t
  8. don’t have to
  9. mustn’t
  10. don’t have to

 

 

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