In order to express the idea of obligation, to talk about rules, duties, or orders, to give strong advice to other people or to ourselves, must, have (got) to, shouldand ought toare used.
See Also
Modals of Necessity: Must, Have got to, Have toModal verbs must, should, ought to - English in useexpressing obligation or its lackModal Verbs of ObligationShouldand ought toare used when aspeaker thinks it is agood thing or right that aperson does something. It indicates that aperson has aduty to do something.
Mustis used to order someone to do something, while have to means that there is arule that forces that person to do something.
Must, have (got) toor simply need tois used when it is necessary to do something.
