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You [verb] use your mobile phone while you're driving. It's against the law.

What verb should be used?

  • don't have to
  • needn't
  • mustn't
  • can't

Is can't correct, or only mustn't is correct?
What is the difference?

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Your last sentence needs serious help. Try Is "can't" correct or ... – Ben Voigt Jun 15 '11 at 0:48
Technically, "can't" means that the proposed activity is physically impossible, while "mustn't" means that the activity is prohibited. However, this technical distinction is often ignored. – Hot Licks Feb 14 '15 at 4:09

I mustn't X means that it is imperative that I not do X.

I can't X means that it is not possible for me to do X.

Therefore, mustn't is the correct answer.

However, in colloquial speech, people would use the word "can't", since it is implied in the statement that you cannot use your mobile phone without breaking the law.

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Notice the lack of parallelism here: cannot = "not possible to do", may not = "not permitted to do", need not = "not required to do", but must not = "required to not do". – Ben Voigt Jun 15 '11 at 13:00
-1 Mustn't isn't the only possible answer to the question, you contradicted yourself when you claim that "can" is used in colloquial speech, ( which is true) so what's to stop it from being written? – Mari-Lou A Jun 10 '13 at 15:20
In some contexts, can and can not refer to physical impossibility, while must not and may not refer to unacceptable acts. Colloquially can not is used for this, too. But the traffic laws will use phrasing like "must not use a cellphone", not "can not use a cellphone". – Andrew Lazarus Jun 10 '13 at 16:27
@AndrewLazarus Can has different meanings; ability, possibility and permission. How would you define the meaning of: "Can I park here?" You could interpret in all 3 manners but normally you would be asking if parking was allowed. The same reasoning applies to the negative form:"I can't park here". Therefore it is common to hear and read: "I can't use my mobile here." – Mari-Lou A Jun 11 '13 at 5:24
@Mari-LouA, Sure, usually. But I have been known to ask my passengers "Can I park there?" in the sense of "Can I maneuver the car into that small space?" The passengers can see that it is a permitted parking place, and they understand that I am referring to my skills, not permission. (I'm not great at parking a car.) The point of my comment is that words like can, must, and shall often take on a more specific meaning in regulatory or legal contexts. I am not sure if you disagree. – Andrew Lazarus Jun 11 '13 at 6:20

If I had to choose a word from that list I would opt for "mustn't".

Luckily, I don't have to because I would say something completely different i.e: "Don't use your mobile while driving. It's against the law."

"You mustn't use your mobile phone while you're driving. It's against the law."

It suggests that I am in this possible situation:

I am driving in my car with my mother sitting next to me. My mobile rings and instinctively I reach to answer it. My mother looks at me sternly (or with deep concern) and tells me: "You mustn't use your mobile etc."

I am like a child being told what to do by her parent, in this case her authority is greater than mine. Must is often used by individuals whose authority is greater than ours, (I am simplifying here but it's generally true) and is also used for giving emphatic advice:

"You must take more exercise. Join a gym."

However, can't is also acceptable if the situation changes:

"You can't use your mobile phone while you're driving. It's against the law."

This time I am driving in my car but I have a friend sitting next to me, and (s)he reminds me that I am breaking the law. (S)he does not have any authority over me, we are equal.

We use "can" and "cannot" to talk about having permission to do or not do something.

"I can't use my mobile" can mean I am not allowed; it is not allowed by the police/the law or it is not right thing to do.

Don't have to and needn't would be inappropriate, and have completely different meanings from can and must.

can and must

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Oxford Dictionaries Online says that can't is simply a contraction for can not, and the entry for can includes this definition and examples:

2 be permitted to:

  • you can use the phone if you want to
  • nobody could legally drink on the premises

The word is certainly used that way in conversation all the time, and sometimes in the news. But not too often. It's more common for an attention-grabbing headline to use the word can't above an article that instead uses more formal words like prevent, prohibit, has no authority, or cannot legally.

Oxford Dictionaries Online includes this usage note:

Is there any difference between can and may when used to request or express permission, as in may I ask you a few questions? or can I ask you a few questions? Many people feel that can should be reserved for expressions denoting capability, as in can you swim?, rather than for those relating to permission. May is, generally speaking, a politer and more formal way of asking for something, and is the better choice in more formal contexts.

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Correct completions for that blank would be "may not" (the abbreviation "mayn't" is very unusual) and "shouldn't". Quick illustration:

May I use my mobile phone while driving?

No, you may not. or "No, you must never use your mobile phone while driving."

Should I use my mobile phone while driving?

No, you shouldn't.

None of the options suggested form any relation to the obvious sense of the sentence in relation to being against the law. "Mustn't" comes closest.

Another good alternative is You must disuse your mobile phone while driving. (Not only may you not stop using it while driving, you must stop if you were already using it.)

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When would you use "mustn't" properly? – Theta30 Jun 15 '11 at 7:15
@Bogdan: "Mustn't" introduces possible confusion. Does "not" modify must, thence "You are not required to use" or "use", thence "You are required to not use". The latter case is desired here, and "You must never use a mobile phone while driving." is far clearer. – Ben Voigt Jun 15 '11 at 12:53

I think in the context of the original sentence,'can't' expresses that using your mobile phone while driving is not allowed, and 'mustn't' expresses necessity to not do it. In this context either can be used, but that's not always the case.

eg. Some people think that home-schooled students can't sit high school exams - 'mustn't' would not fit here.

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