@tiim Ah, yes, the adverbial "the."

@tiim The Youtube person is confused here. Old English "þā X þā Y" meaning "when X then Y" is a real construction, but isn't the origin of adverbial "the" in "the more the merrier". That comes from the Old English demonstrative pronoun (whose main descendant in English today is the word "that"), specifically the form that the demonstrative pronoun took when it was in the instrumental case. So the construction meant something like "by means of more, thus merrier". Screenshot of the Oxford English Dictionary attached so that you know that, unlike the Youtube commentator, I'm not just making this up :-)