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Could you help me please, when I have to use:
must
have to
should?

Thank you,
Fabio
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Anonymousgive more examples,please,explaining the difference bet. must and should
See Must and Should.
Use have to when you want to say that someone has an obligation to do something.
I have to meet Olga at 1:30. (I have an obligation to meet Olga at 1:30.)
CJ
Comments  
must and have to mean the same thing and are both imperative.

should is optional. not as urgent or as important as must/have to

I must breath air or I will die.

or

I have to breath air or I will die.

I should eat something soon because I am hungry.
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give more examples,please,explaining the difference bet. must and should
 CalifJim's reply was promoted to an answer.