Both are used to talk about obligation and in the positive form have similar meanings. In the negative form they have quite different meanings. compare You mustn't tell James (=Don't tell James) You don't have tell George (=You can tell him if you like but it isnt necessary) Must also has no past form. It is expressed by using had to.

This is quite easy to look up: http://www.learn-english-today.com/lessons/lesson_contents/verbs/must_have_to.html http://esl.about.com/od/intermediate-modals/a/Must-Have-To.htm https://www.englishclub.com/grammar/verbs-modals-have-to-must-not.htm

Both give the same meaning , however "Must" is much stronger than " have to" Check this link http://www.englishgrammarsecrets.com/musthaveto/menu.php