VERBS, AUXILIARY. Be, subjunctive and quasi-subjunctiveBe, Beest, &c., was used in A.-S. (beon) generally in a future sense. Hence, since the future and subjunctive are closely connected in meaning, be assumed an exclusively subjunctive use; and this was so common, that we not merely find "if it be" (which might represent the proper inflected subjunctive of be), but also "if thou beest," where the indicative is used subjunctively.
“If, after three days' space, thou here beest found.
And (Mätzner, vol. i. p. 367), bee, beest, bee, pl. bee, is stated by Wallis to be the regular form of the subjunctive. Hence, from the mere force of association, be is often used (after though, if, and other words that often take the subjunctive) without having the full force of the subjunctive. Indeed any other verb placed in the same context would be used in the indicative. Thus:
“Beest thou sad or merry,
The violence of either thee becomes.
“Though Page be a secure (careless) fool, and stands so firmly on
his wife's frailty.
“If Hamlet from himself be ta'en away
And, when he's not himself, does wrong Laertes.
“If he be a whoremonger and comes before him,
He were as good go a mile on his errand.