We can use either must and have to to talk about our obligations and responsibilities.
In the affirmative, the meanings are similar. Both imply that there is an obligation, need or responsibility.
I must get my hair cut today.
I have to get my hair cut today.
The distinction is about autonomy and control. In the first sentence with must the implication is that I have made my own evaluation of my appearance, and made a decision to get my hair cut. In the second sentence with have to, someone else is forcing me to get my hair cut, whether I want to or not.
This ideas is flipped around when we consider someone to have the authority to give us direction or orders. In this case, must is used when someone has the right to impose social consequences (punishment) on us to make us comply. Have to is used to talk about the natural consequences of our actions.
You must get your hair cut. If you don’t meet the company dress code, you will be fired.
You have to get your hair cut if you want to look your best on your date tonight.
The use had to in the Past Tense, to convey both the meaning of must and have to.
I had to get my hair cut last week. My boss made me.
I had to get my hair cut last week. I wanted to look nice on my date.
In the negative, the difference between must and have to is a lot less subtle. Must not is a prohibition. It is a strict order from an authority to not do something, in order to avoid social consequences. Do/does not have to means that something is not necessary. It is up to individuals if they do something or not.
You mustn’t speak during the exam. If you do, you will be asked to leave.
You don’t have to speak during the exam. The test doesn’t evaluate your speaking skills.
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Union rules – Must, have to, mustn’t, don’t have to
