should

Middle English sholde, Old English sc(e)olde; see shall

3. See must¹.

Rules similar to those for choosing between shall and will have long been advanced for should and would, but again the rules have had little effect on usage. In most constructions, would is the auxiliary chosen regardless of the person of the subject: If our allies would support the move, we would abandon any claim to sovereignty. You would be surprised at the complexity of the directions.
Because the main function of should in modern American English is to express duty, necessity, etc. ( You should get your flu shot before winter comes ), its use for other purposes, as to form a subjunctive, can produce ambiguity, at least initially: I should get my flu shot if I were you. Furthermore, should seems an affectation to many Americans when used in certain constructions quite common in British English: Had I been informed, I should (American would ) have called immediately. I should (American would ) really prefer a different arrangement. As with shall and will, most educated native speakers of American English do not follow the textbook rule in making a choice between should and would. See also shall.