Mustn't vs. Don't have to
It is prohibited, it is not allowed. It is important that you do not do something.
You mustn't drink that = It is forbidden to drink that
You mustn't tell him = Do not tell him, you are not allowed to tell him
Don't have to
There is no obligation, you are not required to do something (if you don't want it).
You don't have to drink that = You can drink that if you want, but it' not obligatory
You don't have to tell him = You can tell him, but it's not necessary.