Chapter XXXV On When, Then, and Than
§ 441. The Anglo-Saxon adverbs are whenne and þenne=when, then.
The masculine accusative cases of the relative and demonstrative pronoun are hwæne (hwone) and þæne (þone).
Notwithstanding the difference, the first form is a variety of the second; so that the adverbs when and then are pronominal in origin.
As to the word than, the conjunction of comparison, it is a variety of then; the notions of order, sequence, and comparison being allied.
This is good: then (or next in order) that is good, is an expression sufficiently similar to this is better than that to have given rise to it.