'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.
A special situation exists when a subject seems not to agree with its predicate. For instance, when we want each student to see his or her counselor (and each student is assigned to only one counselor), but we want to avoid that "his or her" construction by pluralizing, do we say "Students must see their counselors " or "Students must see their counselor "? The singular counselor is necesssary to avoid the implication that students have more than one counselor apiece. Do we say "Many sons dislike their father or fathers "? We don't mean to suggest that the sons have more than one father, so we use the singular father. Theodore Bernstein, in Dos, Don'ts and Maybes of English Usage , says that "Idiomatically the noun applying to more than one person remains in the singular when (a) it represents a quality or thing possessed in common ("The audience's curiosity was aroused"); or (b) it is an abstraction ("The judges applied their reason to the problem"), or (c) it is a figurative word ("All ten children had a sweet tooth ") (203). Sometimes good sense will have to guide you. We might want to say "Puzzled, the children scratched their head" to avoid the image of multi-headed children, but "The audience rose to their foot" is plainly ridiculous and about to tip over.