The definite article the of Modern English is the product of the grammaticalization of an Old English demonstrative. The history of this process is roughly as follows:

Proto-Germanic had no article. Old English had no indefinite article, but an incipient definite article. It appears in many, but not all contexts which require the definite article in Modern English ().

The incipient definite article was a distal demonstrative pronoun declining for three genders, sē, sēo, þæt ‘that’. In addition to its exophoric deictic use, it already allowed for discourse-deictic uses (Schmuck 2020: 152), as in both lines of .

In Late Old English and Middle English, the definite and the indefinite articles develop simultaneously. We already find the definite article in Iohannes se Godspellere ‘John the Evangelist’ (Aelfric, De assumptione Beatae Mariae, p. 438). In Middle English, first paradigmatic pressure converts the forms sē, sēo, þæt into þe, þeo, þæt. Second, as gender disappears, these merge into an invariable form þe, the definite article. It is already required in front of the noun world in a context similar to the one of ().

(Dean, James D. (ed.) 1996, Medieval English political writings. Kalamazoo, MI: Medieval Institute Publications (TEAMS Middle English Texts))

At the same time, the former neuter form that survives in its function as demonstrative pronoun, so that this grammaticalization process involves a split ().

The definite article has developed an allomorphy, viz. /ðə/ before consonants, /ði/ before vowels, which is matched by a similar allomorphy in the indefinite article, but is not known otherwise in the language system. Since allomorphy is completely determined by low-level rules of morphology, it is a concomitant of increasing grammaticalization.

In the course of its grammaticalization, the loses precisely that semantic feature which makes it a non-proximal demonstrative, so that only the definiteness remains. In the contexts of its distribution, the article is now largely obligatory. One of the contexts where it is not omissible and only replaceable by other determiners is in front of a noun phrase modified by a superlative adjective or an ordinal numeral, such as the fastest horse or the second son (ch. 4.4). This high degree of obligatoriness is responsible for its frequency: the is the English word with the highest text frequency. Moreover, its position at the start of the noun phrase is fixed. It can only be preceded by quantifiers such as all, both, half, double.

However, even a definite article may proceed in grammaticalization (Schmuck 2020). The English definite article is currently limited to individual (or countable) concrete common nouns whose referent is (presupposed to be) available in any of the anchorage spaces. In other Germanic languages, it also appears in noun phrases with generic reference and with abstract and mass nouns.

References

Himmelmann, Nikolaus P. 1997, Deiktikon, Artikel, Nominalphrase. Zur Emergenz syntaktischer Struktur.. Tübingen: Max Niemeyer (Linguistische Arbeiten, 362).

Schmuck, Mirjam 2020, ‘The grammaticalisation of definite articles in German, Dutch, and English: a micro-typological approach’. Berlin & New York: De Gruyter, Open Access.