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What is the difference between "must" and "have to"? Could you please descripe?
Mar 2, 2018 1:26 AM
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Everything Nicholas said is correct, but there are a few additional layers.

100 years ago, "must" was much more common than "have to".  Since then, in both England and America, "must" has declined, and "have to" has increased. Personally, I almost never use "must" to express obligation in speech: I *always* use "have to".  The same is true for most Americans.  In America, "must" is now mostly used to show *inference* instead of *obligation*--we use it to show that we have arrived at a conclusion based on evidence.

EX: "You walked 20 miles!? You must be tired."// "If you did that for him, you must really love him."// "1000 people took that test, and Sam got the highest score.  He must be smart, diligent, or both."

There are some relict uses for the old meaning of "must".  It appears in technical manuals, rulebooks, and signs.

EX: [sign]"Employees must wash hands before returning to work."//[technical manual]"This machine must be serviced at least once a month, or the warranty will be void."

TL;DR: In modern America, "have to" is almost always a better choice than "must".  If you want to say "mustn't", choose "can't" or "shouldn't" or "are forbidden to" or "aren't allowed to" instead.

March 2, 2018
Both Must and have to express obligation or necessity, but there are some small differences:

•  Must expresses the speaker's feelings, whereas have to expresses, above all, an impersonal idea:
You must come. You are obliged to come (I require that you come)
You have to come. You are obliged to come. (There's a rule requiring you to come)
Must I wear this tie? Am I obliged to wear this tie? (What do you think?)
Do I have to wear this tie? Am I obliged to wear this tie? (Is there a rule about ties?)

 Have to mainly expresses general obligations, while must is used for specific obligations:
I have to brush my teeth twice a day.
I must tell you something.

Important: To express obligation, duty or necessity in the future or the past, must and need are not used. They are replaced by have to:
We must (need to) buy another ticket.
We had to buy another ticket yesterday.
We'll have to buy another ticket later.

However, in their negative forms, mustn't and don't have to have completely different meanings:

•  Mustn't expresses prohibition
You mustn't drive. You are prohibited to drive. You are not allowed to drive.

•  Don't have to expresses the absence of obligation or necessity:
You don't have to drive. You are not obliged to drive (but you can if you want to).
March 2, 2018
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