MODAL IN THE PAST FORM

Modal auxiliary verbs give more information about the function of the main verb that follows it.
Although having a great variety of communicative functions, these functions can all be related to a scale ranging from possibility (can) to necessity (must). Within this scale there are two functional divisions: one concerned with possibility and necessity in terms of freedom to act (including ability, permission and duty), and the other (shall not included) concerns itself with the theoretical possibility of propositions being true or not true, including likelihood and certainty: must = absolute (often moral) obligation, order, requirement, necessity; can/could = physical or mental ability; may/might = permission, option, choice; will = intention in 1st person, volition in 2nd and 3rd persons; and shall/should = in 1st person objective though not moral obligation, no choice, as in: One day I shall die: we all shall die one day; in 2nd and third persons shall implies an incumbent obligation, destiny (It shall come to pass) or a command, decree, necessity imposed by the speaker, as in: A meeting shall take place on the last Friday of every month or a promise, namely that the speaker is stating his obligation to another party that an action or event take place, as in: You shall go to the ball, Cinderella. However, if a speaker states: I will let you go to the ball, Cinderella, in stating his intention, he is, in this instance, also making a promise.