Is "must" more used than "have to" in written language?
Solution 1:
I am not sure about the exact grammatical meaning of each expression, but in everyday American English, "must" and "have to" are used interchangeably for both internal and external obligations. For example, "To obtain a driver's license, you must be at least sixteen years of age" and "I have to help my friend" both sound correct to me, yet they contradict the usage that you were taught.