Must not vs. Don't have to

Both must ________(base verb) and have to _______(base verb) are used for important obligations.  They mean the same thing when used in the positive.

Don't have to ____(base verb) = no obligation -- The situation is not important.  You can, but it's not necessary.

Must not ______(base verb) means DON'T DO IT.  You have an important obligation NOT to do it.

Mustn't = must not.  Mustn't is very common in British English.  In the USA, must not is usually two words

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