Past subjunctive Vs. Present Subjunctive

Question

The verb be is the only one that has a past subjunctive form. In some cases, be, as a subjunctive, retains its present form even if the sentence is in the past tense.

Example:

Present: It is essential that she be present.

Past: It was essential that she be present.

However, in other cases, namely, conditional sentences and subordinating conjunctions, we use the past form of the subjunctive

Example:

If there were a death penalty for corporations, Enron may have earned it.

So two questions here:

1. Is there a reason the subjunctive be remains in its present form in some past tense constructs, yet is converted to the past form were in some others?
2. And if we use the were-subjunctive to express contrary-to-the-fact clauses, then why Shakespeare said this?

If music be the food of love, play on.

Answer 1

Short answer:

1. It switches to were for "wish" statements because were is acting as a marker for such statements, rather than an actual past tense verb.
2. Comparisons between middle/early modern English and today's English are tough to make, as no language is static. We've consistently been losing the case and inflection system from English for centuries (When was the last time you saw "Olde Shoppe" when it wasn't an obvious throwback?). As a result, what is proper during Shakespeare's time, and proper now, are two very different grammar codes for writing.

Long answer:

Most languages, especially older languages have a number of cases (nouns) and inflections (verbs) that English syntax has subsumed. For instance, in biblical Hebrew, A number of Greek variations, Latin, etc, the difference between the subject and predicate noun is clearly marked (not always so in biblical Hebrew, but still pretty common). Specifically to this question inflections changed in verbs creating the Subjunctive (possibility) and the Optative (slight possibility) tenses, in Greek; the Jussive (expression of command) and the Cohortative (exhorting a person to do something) in Hebrew; etc. In English, these have mainly fallen away, replaced with sentence placement and modals, though some are still dependent upon case-forms.

So, a sentence based on a modal and syntax might be, "We could go to the store today." It is based on syntax/position because this (admittedly stilted sentence) "Go to the store today, could we?" is now asking for permission, even though the same words are used without changes tenses.

For nouns, we can see it in: "Who is going to the store," and "You are going to the store with whom?" is an example of "who" changing case form—from subject to object.

For verbs dealing with subjunctives, were has become a condition marker for what is essentially the Optative case—remote possibility: "if he were to win the lottery, I still wouldn't go out with him." Don't think of it as past tense anymore. While it uses the past tense of to be in the plural, it's now a marker for a different function of language.

Why then, do we continue to use the present tense in some subjunctive sentences? That is dependent upon the action within the apodosis (second part of the conditional sentence). To use your sentences:

"It is essential that she be present." "It was essential that she be present."

The past tense is expressed in the protasis. But the action itself was stative. You're stating a fact that has no ending. A past tense means that the action has been completed. Thus, "she was present" could be used if she walked into the room, thus completing the stative verb. However, it would no longer be a subjunctive, because a subjunctive is not completed action. The present tense must therefore be used to keep the action open.

At least, that's how I see it.

Answer 2

As user62235 suggested, the idea that "were" represents the past subjunctive is questionable.

The modal use of "were" in conditional sentences is sometimes called the irrealis "were" to distinguish it from the classic subjunctive, where a subjunctive verb (taking the same form as the infinitive) is used instead of the present indicative in a subordinate clause. E.g., "It is important that you be on time" and "It is imperative that he seek medical attention immediately." Note the absence of the conditional "if" and "would" in these sentences. In modern English, the irrealis "were" is limited almost exclusively to conditional sentences. Note also that the subjunctive verb in each example refers to possibilities in the future rather than the present. We could change the second example to "It was imperative that he seek medical attention." (This sentence is unusual but not wrong.) In this case, the subjunctive verb refers to possibilities in the past. The point is that the classic subjunctive is essentially tenseless. Note that the infinitive, whose form the subjunctive shares, is also tenseless.

Conversely, the irrealis "were" takes the same form as the plural simple past. However, it's used to refer to unreal situations in the present. In the sentence "If I were rich, I'd travel the world," the speaker is not referring to some time in the past. To refer to the past, you'd use the past perfect: "If I had been rich when my son John was growing up, I would have sent him to the best private schools." The past perfect is generally used to refer to an event in the past that preceded another event in the past, but this is not the case here. Thus, in conditional sentences, the name of the verb tense (form) in the subordinate "if" clause doesn't correspond to the actual tense (time). You might say the tense is one step behind the time. Because the irrealis "were" doesn't refer to the past, calling it the past subjunctive is misleading.