aux.v. Past tense of shall
1. Used to express obligation or duty: You should send her a note.
2. Used to express probability or expectation: They should arrive at noon.
3. Used to express conditionality or contingency: If she should fall, then so would I.
4. Used to moderate the directness or bluntness of a statement: I should think he would like to go.
Usage Note: Just as they ignore the traditional rules governing the use of shall and will, Americans largely ignore the traditional rules governing the use of should and would. The two verbs are not always interchangeable, however. To express duty or obligation, should is required and functions as the equivalent of ought to: I (or you or he) should go. But would (and not should) is used to express willingness or promise (I agreed that I would do it) and to express habitual action in the past (In those days we would walk along the canal at night). Would also has the advantage of being a polite substitute for will in requests: Would you lend me a dollar? Either should or would can be used in the first person to express the future from the point of view of the past, but one should bear in mind that should sounds more formal than would: He swore that I should (or less formally, would) pay for the remark. The same principle applies to the verb in sentences that express a hypothetical condition or event: If I had known that, I would (or more formally, should) have answered differently. In the second and third persons, however, only would is used: She assured us that she would (not should) return. If he had known that, he would (not should) have answered differently. · Choosing which verb to use in conditional clauses, such as those beginning with if, can be tricky. In certain clauses, should is used for all three persons: If I (or you or he) should decide to go, we will need a larger car. If it should begin to snow, we will stay here tonight. Would is not acceptable in these if clauses, but it does appear in other kinds of conditional clauses: He might surprise you if you would give him a chance. The best advice is to follow what sounds most natural. When in doubt, writers can try a verb form in the indicative (if it begins to snow) or the subjunctive (if you were to give him a chance). See Usage Notes at if, rather, shall.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.