Lack of necessity and prohibition: Have to and Must in the negative

Examples:

Lack of Necessity

(a) Tomorrow is a holiday. We don't have to go to class.
(b) I can hear you. You don't have to shout.

When used in the negative, must and have to have different meaning.
Do not have to = lack of necessity
In (a): It is not necessary for us to go to class tomorrow because there is a holiday.

Prohibition

(c) You must not look in the closet. Your birthday present is hidden there.
(d) You must not tell anyone my secret. Do you promise?

Must not = prohibition (Do not do this!)
In (c): Do not look in the closet. I forbid it. Looking in the closet is prohibited.
Negative contraction: mustn't. (The first "t" is not pronounced.)

Note: Lack of necessity may also be expressed by need not + the simple form of a verb: You needn't shout. The use of needn't as an auxiliary is chiefly British other than when it is used in certain common expressions such as "You needn't worry."


Cheers!
Naeem