Estas dos páginas al hablar de los verbos modales habla de must y have to.

http://www.shertonenglish.com/resources/es/main-verbs/must.php

http://www.curso-ingles.com/gramatica-inglesa/verbos-modales-perfectos.php

Y esta para enlaces a ejercicios

http://www.agendaweb.org/verbs/modals-must-have-to-exercises.html

Un video

 http://www.youtube.com/watch?v=OibFvGSMwjg (duración 5:35)

2 explicaciones y una aclaración.

1ª Explicación.

"Must" y "Have to" expresan una obligación. A veces se pueden utilizar indistintamente (especialmente con la primera persona donde prácticamente tienen el mismo sentido), no obstante existen ciertas diferencias.

"Must" se utiliza con el tiempo presente y futuro

"Have to" con pasado, presente y futuro

"Must": el emisor impone una obligación.

"Have to": el emisor no impone ninguna obligación, se limita a comunicar una obligación que existe.

(Teacher) You must do your homework (El profesor impone el deber de hacer los deberes)

(Student) I have to do my homework (el estudiante se limita a comunicar una obligación que le ha sido impuesta)

You must stop making noises (El profesor ordena a sus alumnos)

You have to stop smoking (La mujer le recuerda a su marido una obligación que le ha impuesto el medico)

I cannot stay longer, I have to go home (mis obligaciones familiares me obligan a volver a casa)

"Must" también se puede utilizar para expresar un consejo de forma enfática.

Ver la diferencia entre:

You should stop smoking

You must stop smoking

En ambos casos no se trata de una obligación sino de un consejo. Con "must" el consejo es más enfático.

Forma interrogativa

Must + sujeto + verbo principal ?

Do + sujeto + have to + verbo principal ?

Must you leave now?

Do you have to leave now?

Forma Negativa

Cuando la oración es negativa el significado de estos dos verbos es diferente:

"Mustn't": no se debe hacer algo

"Don't have to": no es necesario hacer algo

You mustn't go to that meeting (no debes ir ya que no has sido invitado)

You don't have to go to that meeting (no es necesario que vaya; no se va a tratar nada interesante)

 

2ª Explicación.

 

Must/have to, mustn’t/don’t have to

1 We use must when the speaker thinks it is necessary or important to do an action:

You must go. (= It is important that you go.)

We make negatives, questions and short answers like this:

You mustn’t go.

Must you go? ~ Yes, I must.

2 We use have to talk about an action that is necessary because of rules or laws, or because

someone obliges us to do it:

Doctors sometimes have to work on Sunday.  (It is in the rules of their work.)

We make negatives, questions and short answers with a form of do:

Teachers don’t have to work on Sunday.

Do you have to work today? ~ No, I don’t.

3 positive

In positive sentences we can often use must and have to with little difference in meaning, because many things are important both because we think so and because there are rules:

You must work hard in order to succeed      (or … you have to work … ).

4 negative

Note the difference in meaning between mustn’t and don’t have to.

In negative sentences we often use mustn’t to say that something is against the rules, or against the law:

You mustn’t smoke on buses. (Smoking is against the rules.)

In football you mustn’t touch the ball with your hands. (Touching the ball is against the rules.)

We use don’t have to say that people are not obliged to do something:

In Britain, people don’t have to carry a passport with them. (= People are not obliged to carry one.)

Nowadays pupils do not have to learn Latin at school. (= They are not obliged to learn it.)

5 questions

In questions we usually use do/does … have to (not must) to ask if something is obligatory or important:

Does Michael have to get up early tomorrow?

Do we have to wait here?

 

Una aclaración

Cuando se desea expresar en inglés obligación, ¿hay alguna diferencia entre "have to" y "must"?

 

We use must and have to to say that it's necessary to do something. Sometimes it doesn't matter which you use.

But there is a different between must and have to and sometimes this is important:

MUST is personal. We use must when we give our personal feelings.

you must do something = I (the speaker) say it's necessary:

ex: She's a really nice person. You must meet her.

HAVE TO is impersonal. We use have to for facts, not for our personal feelings.

You have to do something, Because of a rule or the situation:

ex:My eyesight isn't very good, i have to wear glasses for reading.