What is the difference between “can't” and “mustn't” in the expressing of prohibition?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.)