re adverbial ‘the’

monkeytypist:

Does this derive from the times when þon was an instrumental in Old English?  I wonder.

Etymonline for the says:

late O.E. þe, nominative masculine form of the demonstrative pronoun and adjective. After c.950, it replaced earlier se (masc.), seo (fem.), þæt (neut.) [….] Adv. use in the more the merrier, the sooner the better, etc. is a relic of O.E. þy, originally the instrumentive case of the neuter demonstrative þæt (see that).

Of course, I don’t know what any of þose words means.