`  The Heritage Illustrated Dictionary of the English Language—International Edition (1975), in its ``Usage`` note for ``should``, includes the following statement. *Should, in indicating obligation or necessity, is somewhat weaker than ought and appreciably weaker than must and have to. As I understand these four expressions, they all can denote obligation, as when an army officer commands a soldier. *You should intercept this tea. *You ought to intercept this tea. *You must intercept this tea. *You have to intercept this tea. Also, they all can denote recommendation, as when a person speaks to a friend. *You should try this tea. *You ought to try this tea. *You must try this tea. *You have to try this tea. Intermediately, they can denote quasi-obligation or quasi-recommendation, as when a physician issues a ``command`` or a ``recommendation`` to a patient. *You should try this tea. *You ought to try this tea. *You must try this tea. *You have to try this tea. Also, they all can denote probability, as when a person expresses a conclusion about what seems to be likely (and not necessarily desired). *This tea is very good, so it should be very expensive. *This tea is very good, so it ought to be very expensive. *This tea is very good, so it must be very expensive. *This tea is very good, so it has to be very expensive. In my understanding, the four expressions differ in their intensity, but their meaning varies according to their context.  —  `