If a child says, "If I be good, will you buy me an ice cream", is the child being pretentiously archaic, or illiterate?
If a pre-schooler says it, obviously it's illiteracy. Which is fine -- the parents should just correct them.
If a twelve-year-old says it, or more likely writes it, it's pretentious archaism. The teacher should strike it out, quickly.
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PS. Questions about the present subjunctive seem frequent enough to warrant an attempt at prescribing rules for it to follow in our pretentious moments. ;-) What follows is no more than my sense of the old usage.
Its basic meaning is a hypothetical that is not necessarily counter to reality. (The past subjunctive indicates something that is counter to reality but still possible, and the past perfect subjunctive indicates something unreal and no longer possible.)
In consequence, it is generally accompanied by a statement of what is necessary to make the hypothetical, real.
So:
Though he be a good man, he should still keep silent. "Even in the case he really is a good man, present conditions make it impossible for him to speak out." Contrast with
Though he's a good man, he cannot say a word -- in which his goodness is not in question. The "should" is a conditional of "shall". "Must" is also possible, or "must needs" to parrot Shakespeare -- see below.
If I be earnest, I shall be rewarded. The reward is a necessary consequence of my diligence, thus "shall".
If I be in love, I will needs stay at her side. In case I really do love her, I shall want never to leave her. "Will" here denotes my desire, and the aspect of necessity should be conveyed by the adverb "needs", which means "necessarily". Without the adverb of necessity, there's an apparent discordance between the hypothetical and the necessary consequence -- since will is free, not forced.
What I'm getting at is that people who say "if I be" had better follow it up with a "shall".
