theyoudaoicibaDictYouDict[the 词源字典]
the: [OE] The nominative forms of the Old English definite article were se (masculine), sēo (feminine), and thæt (neuter – ancestor of modern English that). In the late Old English period se was replaced by the, probably an eroded version of that and perhaps the same word as the Old English relative particle the. Its drafting in to take the place of se was no doubt promoted by the fact that all the inflected forms of the Old English definite article (thone, thæm, thæs, etc) began with th-. When the distinction between genders began to die out in the early Middle English period, the took over as the general form.
=> that, then, there, this, though[the etymology, the origin, 英语词源]
theyoudaoicibaDictYouDict
definite article, late Old English þe, nominative masculine form of the demonstrative pronoun and adjective. After c.950, it replaced earlier se (masc.), seo (fem.), þæt (neuter), and probably represents se altered by the th- form which was used in all the masculine oblique cases (see below).

Masc. Fem. Neut. Plural
Nom. se seo þæt þa
Acc. þone þa þæt þa
Gen. þæs þære þæs þara
Dat. þæm þære þæm þæm
Inst. þy, þon -- þy, þon --


Old English se is from PIE root *so- "this, that" (cognates: Sanskrit sa, Avestan ha, Greek ho, he "the," Irish and Gaelic so "this"). For the þ- forms, see that. The s- forms were entirely superseded in English by mid-13c., excepting a slightly longer dialectal survival in Kent. Old English used 10 different words for "the" (see table), but did not distinguish "the" from "that." That survived for a time as a definite article before vowels (that one or that other).

Adverbial use in the more the merrier, the sooner the better, etc. is a relic of Old English þy, the instrumentive case of the neuter demonstrative (see that).