As far as I know, When you speak about yourself and you refer to your own feeling of necessity of doing something you use must and not to have to. If I am talking about outside command or necessity I should use have to. But what when you are talking about someone else (the third person singular or plural)? Eg. She has to take this pill, She has to go to doctor, she has to study better, etc which would be a better choice 'must' or 'have to'? How to show that you are talking about either one's own feeling of necessity or about outside command when you speak about a third person and not about yourself?
I must be at the meeting at 10:10. (I require myself-must should be used and not to have to) I have to be at the meeting. (I am required to be there by someone else, only to have to and not must shoud be used here)
My textbook says that when you are not talking about your opinion you shoudn't use 'must'. When you refer to a fact have to is used but not must. At the same time I came across this example; 'Students must pass an entrance examination to study at this school.' This is not someone's opinion, but a fact and if so have to should be used here instead of must I would be grateful for explanations.