1. I must brush my teeth. - anyone, most likely some disobedient child.
2. I have to brush my teeth. - a patient of dentist's, a human with diseased tooth.
3. I must tidy my room. - anyone, most likely some disobedient child.
4.I have to tidy my room. - anyone, probably a child.
5. I must take the medicine. - someone with easy form of illness.
6. I have to take the medicine. - most likely, a regular patient