I want to know the exact difference between "must" "and "have to".
Do you have Michael Swan's "Practical English Usage"? If not, I advise you to buy it as soon as it's possible. Open the book at page 336 and read "must and have to".
Well, I'll try to write a few lines.
In statements about obligation with must the obligation most often comes from the speaker and in questions, from the hearer. To talk about an obligation that comes from outside we usually prefer have to. Compare:
I must work hard. I want to pass my exam.
Catholics have to go to church on Sundays.
For a future and a past obligation have to is used only.
you've given the gist of the two forms, and that was the point here, I presume . However, this formal distinction between Have to and Must entails many distorsions in daily conversations. Sometimes, the nuances are so slight that those values are quite blurred . Catholics have to go to church, Protestants must . An instance for introspection and meditation .