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Modal Verbs of Obligation

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We can use have to + infinitive, must + infinitive and should + infinitive to express obligation (something you have to do).

|Present |Positive |Negative |

|have to / |strong obligation (possibly from outside) |no obligation |

|don't have to |Children have to go to school. |I don't have to work on Sundays. |

| |(sometimes 'have got to') | |

| | |You don't have to eat anything you don't like. |

|must / mustn't |strong obligation (possibly based on the speaker's |negative obligation |

| |opinion) |You mustn't smoke here. |

| |I must study today. | |

|should / shouldn't |mild obligation or advice |mild negative obligation or advice |

| |You should save some money. |You shouldn't smoke so much. |

Be careful about the difference between mustn't and don't have to!

Mustn't means it's not allowed, or it's a bad idea:

• You mustn't eat so much chocolate, you'll be sick

Don't have to means you don't need to do something, but it's fine if you want to do it:

• I don't have to get up early at the weekend(of course, if I want to get up early, that's fine, but I can stay in bed if I want).

|Past |Positive |Negative |

|had to / didn't have to |obligation in the past |no obligation in the past |

| |I had to wear a school uniform when I was a child. |We didn't have to go to school on Saturdays. |

|must* |changes to 'had to' |- |

|should have + pp / |a past action which didn't happen: the advice / |a past action which didn't happen: the advice / |

|shouldn't have + pp |regret is too late |regret is too late |

| |You should have gone to bed earlier, now you have |You shouldn't have taken that job., it was a bad |

| |missed the train. |idea. |

* Remember 'must have done' is a modal verb of deduction or speculation, not obligation in the past. For example: Julie must have left. Her coat's not here. See modals of probabilty for more information.

Should / Must / Have to

Should

Se utiliza para dar consejos. Se indica que algo es bueno y por lo tanto se debería hacer. Es una recomendación, no una obligación. Tiene menos fuerza impositiva que "must" y que "have to".

• If you feel bad you should go to the doctor

También se utiliza para dar una orden pero de forma muy educada.

• Anyone who wants to enter the conference should show his credentials

(Se trata realmente de una obligación -es obligatorio mostrar las credenciales para entrar en la conferencia-, pero está expresada de forma poco coercitiva)

Construcción: este verbo modal va seguido del infinitivo del verbo principal sin la partícula "to".

Se utiliza en pasado, presente y futuro.

• I should have finished my report yesterday (pasado)

• You should help me, I can not do it alone (presente)

• Tomorrow you should call your parents (futuro)

El pasado se utiliza para indicar algo que se debería haber hecho y no se hizo.

La forma negativa es: shouldn't

• You shouldn't smoke

Y la forma interrogativa: Should + sujeto...?

• Should I wear a dark suit for the party?

 Must / Have to

Expresan una obligación, algo que hay que hacer.

• You must come here immediately (es una orden)

• If you want to make a career in a company you have to speak English (No hay alternativa: o hablas inglés o no hay carrera posible)

Comprueba la diferencia con la siguiente oración:

• Nowdays it is very important to speak English. You should do it.

Aquí se trata de un consejo, mientras que en la anterior era una obligación.

Por otra parte, cuando se utiliza "must" / "have to" el emisor tiene la convicción de que la obligación va a ser cumplida, mientras que cuando se utiliza "should" no se sabe si el consejo se va a seguir o no.

• We should buy a new car (es un deseo; no hay seguridad de que vayamos a hacerlo)

• We must buy a new car (hay muchas probabilidades de que lo hagamos)



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