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I was doing some exercises with must and have to.

I learnt that "have to" have to be used with laws or obligations, something you must do because of a external factor.

But in these exercises they use "must" to say a obligation about traffic signs, what I think must be "have to".

For example:

"You have to turn left." is replaced by "You must turn left."

Is this choice because many laws have the negation option?

So, by convention they use "must" to negation be implicit like "mustn't" and not "don't have" what may cause some misunderstoods.

Or this is because we suppose that sign is "speaking" with us?

Can I use "You have to turn left" instead of "You must turn left" ?

Thank you.

  • 'ought to' is better. You ought to turn left here – Leo Dec 10 '14 at 16:40
  • @Leo "Ought to" is weaker than "must" and "have to"; to me at least it implies it's something you should probably do, but isn't completely necessary. Like if a friend were navigating for you, and they saw that a certain route had less traffic, they might say "you ought to turn left here". It's not something I'd expect to mean "going straight on is illegal"! – Muzer Mar 20 '17 at 16:24
  • As a British English speaker, I'm not aware of the rule of "must" vs "have to" to which you're referring. For example, see the UK's Highway Code making extensive use of "must" to mean "are required to by law": gov.uk/guidance/the-highway-code/… – Muzer Mar 20 '17 at 16:26
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There's a good reason to use must: the sentence is less likely to be confusing on the first read.

The word have is a multi-purpose word, and saying "You have..." at the start of a sentence can get the reader thinking in the wrong direction, if only momentarily.

Consider:

  • You have some ketchup on your cheek.
  • You have two messages in your inbox.
  • You have a cold; your head is all stuffy.
  • You have an appointment at two o'clock – don't be late.
  • You have no idea how much I want to ski in the Alps.
  • You have heard it said, "Do not steal."
  • You have it so good – stop complaining!
  • You have three sons, right?
  • You have to eat breakfast before you go to school.

With have having so many uses and meanings, I think must is the better word for a traffic sign, where we expect drivers to glance at a sign and immediately grasp its meaning.

In short:

YOU HAVE TO TURN LEFT

and:

YOU MUST TURN LEFT

have the same meaning, but the second is more direct and concise, making it the better choice for a traffic sign.

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Very interesting question.

Personally, I cannot imagine either of these occurring on a traffic sign, because traffic signs are typically short. They would just say "Turn left" or "Left turn only" or "Left turn required."

In my opinion, "must" and "have to" are synonyms, except that "must" (when it implies an obligation) is rare in informal everyday language. Although I don't find "have to" especially informal, I can see where you'd pick the more formal one for a road sign.

But this is really a stretch. "You have to turn left" and "You must turn left" would mean the same thing on a road sign.

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    I wrote something here which I think will help you, too, although it has some stuff you already know. ell.stackexchange.com/questions/26784/must-and-should/… – hunter Dec 10 '14 at 15:59
  • I see, I forgot to say I refer to traffic signs without words, just the picture. – Apprentice Dec 10 '14 at 16:06
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    Oh I see, you're TALKING about the traffic sign. Then you would usually say "have to." It would be weird, but correct, to say "must." – hunter Dec 10 '14 at 17:31

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