MUST / MUST NOT (Mustn't)

You MUST do something  =  it is necessary that you do it

(because it is an Obligation).

"Remember, don't tell anybody, you must keep the secret."

According to the doctors she must stay in the hospital until tomorrow.

You must pay attention while driving, it's important!

"Shhh! The baby is sleeping, we must be very quiet!"

You MUST NOT do something  =  it is necessary that you do not do it

(because it is a Prohibition).

"Remember, you must keep it a secret, you must not tell anybody."

According to the doctors she must not leave the hospital until tomorrow.

You mustn't speak on your mobile phone while driving.

"Shhh! The baby is sleeping, we mustn't make any noise!"