本文透過歷時語料探索越南語的能願模態詞（dynamic modals）系統，並大致勾勒出越南語模態詞分佈的時間順序。此外，我們進一步追溯在語料中發現的兩種能願模態詞được的句法－語意變化（syntacto-semantic changes）。本文認為動後（post-verbal）模態詞được是由結果標記（resultative marker）發展而來的，而曾經存在的動前（pre-verbal）模態詞được經過重新分析（reanalysis）由單純的可能性語意（opportunity reading）轉變為受惠性（benefactive）語意。為何在現代越南語中能願模態詞được僅出現於動詞後是一個長期存在的難題，本文藉由歷時的角度為此提出了一個解釋。

This preliminary work examines the Vietnamese diachronic data to first attempt a glimpse into the inventory of specialized dynamic modal elements in Vietnamese, and to adumbrate a rough chronology of this modal system. It further traces back the syntacto-semantic changes resulting in two distinct instances of the dynamic modal được. I argue that post-verbal được is immediately derived from a resultative marker, and there once existed a pre-verbal được with a pure opportunity reading, which was later reanalyzed as được with a benefactive interpretation. The paper thus provides an explanation to the long-held puzzle regarding the sole post-verbal position of dynamic modal được in Present-Day Vietnamese.