Use have to when it is necessary to do something, but it is not your choice to do it.
- My wife doesn’t like long hair, so I have to get mine cut.
(Wife’s decision to get it cut.) - You are not allowed to turn left here. You have to turn right.
(Road signs tell you, so it is not your choice.)
Use must when it is necessary to do something, and it is your choice to do it.
- I hate long hair, so I must get my hair cut.
(My choice to get it cut.) - This cake looks delicious. I must try some.
(My choice to try some.)
