Sooner or later in the study of grammar, students, parents, and teachers will have to address the problem of the subjunctive mood. The term "mood" refers to the attitude of the speaker or writer toward the content of sentences. Grammarians usually distinguish three moods: 1) indicative (statements of fact); 2) imperative (commands); and 3) subjunctive (matters of unreality). These moods are communicated in the form of the verb:

1) indicative -- Bill is good.
2) imperative -- Bill, be good.
3) subjunctive -- If Bill were that good, he would have won the game.

Perhaps the first thing we need to note within the context of KISS Grammar is that the terms "indicative" and "imperative" are not essential. Grammarians and linguists will scream about this, but the fact is that all preschoolers are perfectly competent at handling sentences in indicative and imperative moods. Since the research on the teaching of grammar shows that the real problem is the massive amount of terminology that is usually taught, an insistence on teaching these two terms seems to be illogical. This should be even more apparent when we note that currently, most high school graduates cannot identify verbs in the first place. In that context, what can "indicative" and "imperative" mean to them?
     Second, we should note that the subjunctive is not a matter of sentence structure. In other words, it is not a matter of how words chunk to each other within a sentence to create meaning; instead, it is, we might say, a matter of usage. For practical matters, the question is limited to the form of the verb.
     First and foremost, parents and teachers need to be aware of the subjunctive so that they do not mistakenly "correct" sentences that are already correct. In "If Bill were that good, he would have won the game," the subjunctive "were" is identical in form to the indicative past tense plural. Thus parents and teachers who are not aware of the subjunctive might want to "correct" the sentence to "If Bill was that good, he would have won the game." This is not a major problem, but as Paul Roberts noted

      This form -- were -- occurs mainly in contrary-to-fact conditions. It has been losing ground for centuries in this usage, but there are still a few constructions in which it is nearly universal in all levels except Vulgate. Notable are the clauses "if I were you" and "I wish I were you." Was in these clauses would be strong indication of the speaker's lack of education. (Understanding Grammar, 165.)

     In Choice Written and Spoken English and to some extent in General Written, the present subjunctive persists in noun clauses after verbs of asking, ordering, urging, insisting, etc., and also after adjectives of similar meaning:
     He asked that I come to his office.
     My employer insists that I be prompt.
     It is imperative that he take the next plane. (164)

     The preceding is a simplified version of a complicated question. As usual, Paul Roberts' Understanding Grammar is my favorite grammar textbook because, unlike most textbooks, it acknowledges the complexities:

     Nowhere in the study of grammar is it more important to distinguish notional and formal categories than in the matter of mood, and nowhere is it more difficult. (160)

     Unfortunately, textbooks and linguists rarely address the most important questions: How do students learn the subjunctive? And how much trouble does it give them? Based on more than a quarter of a century teaching writing to college Freshmen, I would say that most students handle it very well, and I am almost positive that they did not learn to do so by studying explanations such as those given in textbooks. They probably learned it in the same way that they learn all the other idiomatic aspects of English.
     This still leaves us with the questions of when and how the subjunctive mood should be taught. As noted, parents and teachers should be aware of it so that they do not confuse it with errors of agreement or of tense. Compared to sentences in the indicative and imperative, subjunctives are very infrequent. As a result, should students in early grades run into them in identification exercises, teachers could, as they do for example with verbals, simply point out that the verb is in the subjunctive mood and that the students will study subjunctives later.
     My guess is that, within the ideal KISS sequence of instruction, students could be formally introduced to the subjunctive in fifth or sixth grade. In that sequence, grades four, five, and six are devoted to recognizing S/V/C patterns. Thus by the middle of fifth grade, most students should be fairly comfortable with the recognition of finite verbs. This determination, however, really needs to be made by individual parents and teachers. Capable of identifying verbs, many of these students would find the subjunctive to be a meaningful concept. Some students, of course, lag behind, and students forget. Thus it might be a good idea, once a year, to do a short formal exercise identifying and writing verbs in the subjunctive mood.

The American Heritage® Book of English Usage. A Practical and Authoritative Guide to Contemporary English.  1996. (at Bartleby.com) 

from The Columbia Guide to Standard American English  by Kenneth G. Wilson

 "When you wish upon a star,  you're wishin' 'I were' not 'was'" from Professor Malcolm Gibson's Wonderful World of Editing.

"The Subjunctive Mood" at Get it Write

"Does English Have a Subjunctive Mood?" by Neil Coffey

"God save the subjunctive !" by  Charles A. Finney

"An outdated grammar rule: the subjunctive. Let's trash it." by Laraine Anne Barker