The existence of a definite article is not shared by the common ancestor of the Indo-European languages. The older languages get along without them; there is no article in Latin, Sanskrit, or in conservative Indo-European languages like Russian. As in the etymologies of many other languages, the word originally entered the language as a demonstrative pronoun or adjective; compare the fate of the Latin demonstrative ille in the Romance languages, becoming French le and la, Spanish el and la, and Italian il and la. The and that are common developments from the same Old English system. Old English had a definite article se, in the masculine gender, seo, feminine, and þæt, neuter. In Middle English these had all fallen together into þe, the ancestor of the Modern English word.
It is generally pronounced with a long e before a word starting with a vowel and with a schwa before a word beginning with a consonant. However, for emphasis on the importance or veracity of its following word, the is pronouced with long e even regardless whether the following word is consonant- or vowel-initial. In this use, the alone may be used to eliminate the adjective "pre-eminent", as in "the hospital for back problems". But in written expressions, such as "the novelist of middle-class despair," it can stand without emphasis since the context is clear.
The can also mean "sufficient", as in "lacked the gumption to make his move". It can also be used as a possessive pronoun, as in "can't walk right since the ankle went"; as an expression of a ratio, as in "five apples the dollar"; and as an adverb.
In its adverbial use, it appears twice, each time before a comparative adjective or adverb to denote a commensurate relationship, as in "the more the merrier".
See A, an.