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'Must' is modal verb most often used to express necessity and certainity, to indicate a strong intention or recommendation, to emphasize an opinion, advise or sentiment and for prohibition (when used negatively).
'Must' is stronger than should and used to show duty or obligation.
"Auxiliary-cum-verbs" are :
These 11 auxiliaries are also used as verbs, therefore, they are called Auxiliary–cum–verbs.
These are used to form Tenses. ( Please refer Tense Table).
"Pure Auxiliary verbs" are :
These 20 auxiliaries only support normal verbs, therefore, they are called Pure Auxiliary Verbs. They are also called Models or Model Auxiliary Verbs.