to-
“that, there (distal deictic)”there/that
PIE demonstrative pronoun meaning that or there. Source of English "the," "that," "there," and definite articles across many branches.
Discussion
*to- is a Proto-Indo-European demonstrative pronoun meaning "that" or "there," functioning as the distal deictic—pointing to what is distant from the speaker.
The root gives Old English þæt "that" (modern English that, the, there, then, than, this) with the characteristic Germanic dental fricative from PIE *t (later reinforced by Grimm's Law patterns). The definite article "the" derives from the demonstrative—one of the most important grammatical developments in English history. German der/die/das, Dutch de/het, and Old Norse þat continue the Germanic forms.
Latin -tus (in is-tus) and the demonstrative formations contain this root. Greek tó "the" (neuter article) preserves it transparently. Sanskrit tád "that" confirms the Indo-Iranian branch.
The root opposes *ḱe- "this/here" (proximal) in the PIE deictic system. While *ḱe- points to what is near, *to- points to what is remote. This binary system is ancestral to the English this/that distinction.
As the source of the English definite article, this is arguably the single most frequently used PIE root in modern English. Modern descendants include the, that, there, then, than, they, them, and this (with demonstrative reinforcement).
Notes
Source of English "the", "that", "there". Distal demonstrative pronoun/adverb.