should

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

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.

before 900; Middle English shal, Old English sceal; cognate with Old Saxon skal, Old High German scal, Old Norse skal; compare German soll, Dutch zal

The traditional rule of usage guides dates from the 17th century and says that to denote future time shall is used in the first person ( I shall leave. We shall go ) and will in all other persons ( You will be there, won't you? He will drive us to the airport. They will not be at the meeting ). The rule continues that to express determination, will is used in the first person ( We will win the battle ) and shall in the other two persons ( You shall not bully us. They shall not pass ). Whether this rule was ever widely observed is doubtful. Today, will is used overwhelmingly in all three persons and in all types of speech and writing both for the simple future and to express determination. Shall has some use in all persons, chiefly in formal writing or speaking, to express determination: I shall return. We shall overcome. Shall also occurs in the language of laws and directives: All visitors shall observe posted regulations. Most educated native users of American English do not follow the textbook rule in making a choice between shall and will. See also should.