'Have to' and 'must' are both used to express obligation. There is a slight difference in the way that they are both used. 'Have to' shows that the obligation comes from someone else, not the speaker. This is usually referring to a rule or law. We have to be at the airport at least two hours before the flight. I have to work on Saturday. They have to wear their uniforms at school. 'Must' shows us that the obligation comes from the speaker. I must hand in my thesis by tomorrow. I really must call my parents.