“Subjunctive: A grammatical category that contrasts with indicative in the mood systems of verbs in various languages, and expresses uncertainty or non-factuality. Some languages have a range of subjunctive tenses: Latin (Caveat emptor: Let the buyer beware); French (Je veux que tu travailles, literally ‘I want that you should work,’ I would like you to work’). There was no such a system in Old English (Ne he ealu ne drince opp w in: Nor shall he drink ale or wine), but in Modern English there are few distinctive subjunctive forms and the use of the the term is controversial. Grammarians have traditionally described English as if it had a subjunctive system comparable to Latin and French, with present and past subjunctive tenses. This approach poses problems, because the ‘present’ subjunctive is used in subordinate clauses referring to both present and past time: They are demanding that we pay now and They demanded that we pay there and then. In form, this subjunctive is identical with the base of the verb (the bare infinitive), which means that, when the reference is to present time, it only differs from the indicative (except with the verb be) in the third-person singular: We suggest that he leave soon as against They say he leaves at dawn tomorrow. With past reference, the difference between the indicative is noticeable for all persons, as in We suggested he leave.
The subjunctive has three uses: (1) Mandative. Mainly in subordinate clauses, following a verb, adjective, or noun expressing a past or present command, suggestion, or other theoretical possibility: I insist that she disband the team; It is essential that it be disbanded; She ignored his request that she disband the team. When a negative is used with this subjunctive, it precedes the verb: He requested that she not embarrass him, except with be when not be and be not are both possible: He was anxious that his name be not/not be brought into disrepute. The mandative subjunctive is commoner in American English than British English, but appears to be on the increase in British English. In both, but especially in British English, it can be replace by a should– construction or an indicative: He requested that she should not embarrass him; He was anxious tat his name was not brought into disrepute. (2) Conditional and concessive. Sometimes formally in subordinate clauses of condition or concession: If music be the food of love, play on…; Whether that be the case or not…; Though he ask a thousand times, the answer is still NO. The alternatives are an indicative or a should-phrase: If music is…; Though he should ask…. This usage does not extend to past time. (3) Formulaic. In independent clauses mainly in set expressions. Some follow normal subject-verb word order (God save the Queen! Heaven forbid!), while others have inversion of the main verb and subject (Long live the Queen!); Far be it from me to interfere). Come plus a subject introduces a subordinate clause: Come the end of the month, (and) there’ll be more bills to pay.
The ‘past’ subjunctive is now often called the were-subjunctive, because this is the only form in which there is a distinction from the indicative, and then only in the first- and third-person singular: If I were you…as opposed to If I was you. It is used with present and future (not past) reference in various hypothetical clauses, including condition: If only I were young again; If he were asked, he might help; This feels as if it were wool; I wish she were here now; Suppose this were discovered; I’d rather it were concealed. In popular and non-formal speech and writing, the were-subjunctive is often replaced by the indicative was, which brings this verb into line with other verbs, where the past tense is similarly used for hypotheses about the present and future: If only I knew how; I’d rather you said nothing. Were is, however, widely preferred in If I were you…. In the fixed phrase as it were (He’s captain of the ship, as it were), were cannot be replaced by was. The use of were instead of was to refer to a real past possibility is generally considered an over-correction: If I were present on that occasion, I remember nothing of it. This contrasts with the purely hypothetical past, If I had been present…, which strongly implies but I was not.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.