The word 'then' originates from the Old English 'þonne', 'þænne', 'þanne', which meant "at that time, when, as, than." This Old English form is derived from the Proto-Germanic 'þana-', a variant of the demonstrative base 'þa-', meaning "that." This base is traceable back to the Proto-Indo-European (PIE) root 'to-', also a demonstrative pronoun meaning "that." The semantic journey is straightforward: from a general demonstrative sense of pointing to something ("that"), it specialized in Germanic to indicate a specific point or sequence in time ("at that time"). Its logical connection to conditional statements ("in that case") and sequence ("next") developed naturally from this core temporal meaning.

The word 'then' is a simplex adverb derived from Old English. It does not contain separable morphemes in its modern English form.