verbs of NECESSITY OR OBLIGATION

Si miráis esta página, equipara explçicitamente NECESSITY y OBLIGATION, como lo mismo:

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/330-modals-necessity.htm

Let's read about how to express necessity or obligation. The modal verbs “must,” “have to” and “have got to” show that something is not optional; it is necessary.

http://advanced-english-grammar.com/modal-verbs-of-necessity.html 

The modal verbs of necessity show obligations in the past, present, or future. It can be a necessary action that was required over and over again, or something that occurred just once.

The modal verbs of necessity are have to, have got to, and must.
 

http://www.youtube.com/watch?v=51fJcZjI2jQ 


http://www.egeacademy.com/blog/grammar-points/modal-verbs-must-have-to-obligation-necessity/

MUST / HAVE TO

OBLIGATION / NECESSITY

HAVE TO

We use “have to” when the necessity comes from outside the speaker or when others decide for the speaker.

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

"Must" indica una obligación, prohibición o necesidad. También puede emplearse "have to" (tener que).



http://www.manuales.com/manual-de/verbos-modales-en-ingles-must 
 
 Tanto “Must” como “To have to” expresan la necesidad de hacer algo importante. Las diferencias de utilización son, prácticamente, inexistentes. No obstante, algunos sectores consideran que las acciones señaladas con “must” están más vinculadas con la voluntad del hablante. Por el contrario, si usamos “have to” estamos ante circunstancias ajenas a esa voluntad y sujetas a otros elementos externos.



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GOLDEN RULES ABOUT MODAL VERBS

• Modal verbs never add an -s in the third person singular: 
• He must work harder to pass his exams

• Modal verbs are followed by infinitive without to except from ought to.
• You should behave better!

• Modal verbs don´t need auxiliares to make negative sentences or questions: 
• Should I apologise?
• I mustn´t smoke in hospitals

• Modal verbs only have one form. You need other verbs to express the same thing in the past, future etc.
• I´ve been able to finish on time.