Where does “-던” come from?

-던 (Korean) comes from Korean -더-, from Old Korean 如 — declarative mood sentence-final verbal suffix.

Ancestry of “-던”, step by step

StepLanguageWordMeaning
1Korean-더-The retrospective suffix, marking that the speaker (in declaratives) or the addressee (in interrogatives) has personally observed the action/state conveyed by the verb/adjective, or a fact from which the action/state could be inferred; that either this action/state was not taken by the observer, or
2Old Koreandeclarative mood sentence-final verbal suffix
Every word from Old Korean Every word from Korean -더-
-던 — Etymology, Origin & Word History | Etymology Explorer