[Modal auxiliaries] Lack of Necessity and Prohibition: have To and Must in the Negative

When used in the negative, must and have to have different meanings.

Lack of necessity
I can hear you. You don’t have to shout.	May also be expressed by need not + the simple form of a verb
Prohibition
You must not tell anyone my secret. Do you promise?
Don’t tell anyon my secret.
You can’t tell anyone my secret.
You’d better not tell anyone my secret.