Must / Have to

Must es un verbo modal que expresa la obligación de hacer algo. 

Como todos los modales, va seguido de otro infinitivo sin “to”. 

Afirmativa 

I must work 

You must work 

He/she/it must work 

We must work 

You must work 

They must work 

Interrogación respuesta breve +  - 

Must I work?                                       Yes, I must No, I mustn´t 

Must you work?                                  Yes, you must No, you mustn´t 

Must he / she / it work?                       Yes, he / she / it must No, he / she / it mustn´t 

Must we work?                                    Yes, we must No, we mustn´t 

Must you work?                                  Yes, you must No, you mustn´t 

Must they work?                                 Yes, they must No, they mustn´t 

Negación: Mustn´t está explicado en otro apartado de los Verbos Modales, por la diferencia de significado con must. · Must también se utiliza para expresar una conclusión: 

- The light is on. He must be at home. 

- It´s very windy and cloudy. It must be cold. 

- He´s got a lot of white hair. He must be over 40. 

- There´re pieces of glass on the road. There must have been an accident. 

Must sólo tiene forma de presente. Por ello, para cualquier otro tiempo se utiliza have to: 

- I´ll have to work next Sunday. 

- She had to clean all the house.

Have to indica obligación, al igual que must. 

Afirmativa 

I have to work                                     we have to work 

You have to work                                you have to work 

He/she/it has to work                          they have to work

Interrogación respuesta breve + - 

Do I have to work?                            Yes, I do No, I don´t 

Do you have to work?                        Yes, you do No, you don´t 

Does he / she / it have to work?          Yes, he / she / it does No, he / she / it doesn´t 

Do we have to work?                          Yes, we do No, we don´t 

Do you have to work?                        Yes, you do No, you don´t 

Do they have to work?                       Yes, they do No, they don´t - 

También puede utilizarse have got to, que es algo más informal.

Existe una pequeña diferencia de significado entre must y have to:

Must indica una obligación que se impone uno mismo, más que las circunstancias: 

- I must study for the exam. 

- He must work more to have enough money. 

Have to, en cambio, expresa una necesidad impuesta por las circunstancias externas: 

- She has to get up at 7 every morning. - We have to work till 6 on Mondays.

Don´t have to expresa que no es necesario u obligatorio hacer algo. forma completa forma contraída

I do not have to work I don´t have to work 

You do not have to work you don´t have to work 

He/she/it does not have to work he/she/it doesn´t have to work 

We do not have to work we don´t have to work 

You do not have to work you don´t have to work 

They do not have to work they don´t have to work 

Ejemplos: 

- You don´t have to do the shopping. I´ll do it tomorrow. 

- I don´t have to go to school at weekends. - They don´t have to wash by hand because they´ve got a washing machine.