CONJUNCTIONS. And or an with the subjunctive"And" with the subjunctive. The true explanation appears to be that the hypothesis, the if, is expressed not by the and, but by the subjunctive, and that and merely means with the addition of, plus, just as but means leaving out, or minus. The hypothesis is expressed by the simple subjunctive thus:
This sentence with and would become, "I must become a borrower of the night and my horse go not the better," i.e. "with, or on, the supposition that my horse go not the better." Similarly in the contrary sense, "but my horse go the better," would mean "without or excepting the supposition that my horse, &c." Thus Chaucer, Pardonere's Tale, 275:
“Go not my horse the better
I must become a borrower of the night.
It is no curtesye To speke unto an old man vilonye But he trespas.So also Mandeville (Prologue):
Such fruyt, thorgh the which every man is saved, but it be his owne defaute.