故
gù · HSK 7
hence
古 (old/cause) + 攵 (action) — old reason leads to a result, hence therefore
故 combines 古 (old, ancient) on the left with 攵 (a strike/action radical) on the right. The old cause produces an action, so 故 came to mean cause, reason, and by extension therefore or hence.
故 — 古 (old) + 攵 (action/strike)
故 combines 古 'old' as the phonetic with 攵, a variant of 攴 meaning to strike or act. Originally it referred to a cause or reason behind an action. From this it extended to mean 'former, old, deceased' and 'therefore, hence' as a connector.