[A] MOTHER: You must wipe your feet when you come in; SMALL BOY: I have to wipe my feet every time I come in. [B] MOTHER: You must wear a dress tonight. You can’t go to the opera in those dreadful jeans; EMPLOYER: You must use a dictionary. I’m tired of correcting your spelling mistakes; DOCTOR: You must cut down on your smoking; You have to wear uniform on duty, don’t you? ; You have to train very hard for these big matches, I suppose; You’ll have to get up earlier when you start work, won’t you? ; You’ll have to cross the line by the footbridge. [C] RAILWAY COMPANY: Passengers must cross the line by the footbridge; OFFICE MANAGER: Staff must be at their desks by 9:00; REGULATION: A trailer must have two rear lamps. In this office even the senior staff have to be at their desks by 9:00; She has to make her children’s clothes. She can’t afford to buy them; They’ll have to send a diver down to examine the hull. A driver who has knocked someone down must stop, (El hablante piensa que es el deber del conductor parar); Something must be done to stop these accidents. [D] TYPIST: I must/will have to buy a dictionary; PATIENT: I must/have to/will have to cut down on my smoking. I have to take two of these pills a day. I must tell you about a dream I had last night; Before we do anything I must find my cheque book. [E] You must come and see us some time, (Esta es la manera corriente de expresar una invitación informal). The children have to play in the street till their parents come home. This sort of thing must stop! (El hablante tiene autoridad o sentimientos intensos al respecto). You must write to your uncle and thank him for his nice present. If your father was a poor man you’d have to work. We have to walk our dog twice a day. NOTICE IN SHOP WINDOW: Closing down sale! Everything must go! [F] I ran out of money and had to borrow from Tom; You had to pay duty on that, I suppose? ; There were no buses so he had to walk.
[A] must expresa una obligación impuesta por el hablante; have to expresa obligación externa. En [B] se dan ejemplos de la «autoridad del hablante» y la «autoridad externa» en la 2ª persona. [C] En la 3ª persona must se usa principalmente en órdenes escritas o instrucciones; si nos limitamos a enunciar o comentar las obligaciones de otra persona usamos have to. Si usaramos must en lugar de have to en los ejemplos con have to de arriba en [C] podría implicar que el hablante tenía autoridad para ordenar estas acciones. Se emplea must cuando el hablante aprueba una obligación; o cuando el hablante siente con fuerza. [D] En la PRIMERA PERSONA la diferencia entre must y have to es menos importante y se utilizan indistintamente las dos formas; pero have to es mejor para hábitos, y must es mejor cuando las obligaciones son urgentes o le parecen importantes al hablante. [E] Agrupa algunos otros ejemplos (en todas las personas). [F] Para expresar obligaciones afirmativas en el pasado se usa had to. La distinción entre la autoridad del hablante y la autoridad externa no puede expresarse porque hay solo una forma: had to.]
A Practical English Grammar (A.J. Thomson & A.V. Martinet) 