'Then' and 'than' were identical until the 17th century — both from PIE *to- (that). Time and comparison split.
At that time; after that; next; in that case; therefore.
From Old English 'þanne, þonne' (then, at that time, in that case), from Proto-Germanic *þanā, from the PIE demonstrative stem *to- (that, the one over there). The PIE demonstrative *to- is among the oldest grammatical words reconstructed for the parent language, serving as the basis for third-person pronouns and demonstratives across the whole family. 'Then' is the temporal form: *to- + a locative suffix pointing to a time rather than a place, just as 'there' points to a place and