「當」是上古至中古重要的情態動詞兼未來時標記，可表達認識情態、道義情態及未來時間定位。由於「當」所表達的情態及未來時語義類型與跨語言研究中的某些情態詞或未來時標記有類似之處，因此，歷來學者多參照跨語言研究所提出的由道義情態向認識情態以及由道義情態向未來時發展之語義發展單向性假設來描繪「當」的情態語義及未來時標記發展。不過，近年來這類單向性假設也受到若干學者的質疑。本文透過實際深入考察「當」在文獻中的語義、用法及發展，重新探討「當」的各類情態語義及未來時發展途徑。本文的結論是，「當」所表達的各類語義都是由動詞「當」的不同語義在不同語境中引申出來，各類情態及未來時語義間不必然有前後引申關係。這個結論雖與一般跨語言的情態及未來時研究不同，卻是上古漢語情態動詞及未來時標記的特色。

Dang ( 當) is an important modal verb in Old and Middle Chinese that expressesepistemic and deontic modality and also a future marker that locates a situationposterior to a time. Since dang has similar functions as some modals and futuremarkers in cross-linguistic studies, the semantic development of dang is oftenexplained with cross-linguistic studies’ hypotheses that both epistemic modalityand futurity are developed from deontic modality. However, the unidirectionalhypotheses have been challenged in recent years. This paper reinvestigates thedevelopment of dang’s modal and future meanings by examining its usages in Oldand Middle Chinese texts, and concludes that dang’s various meanings in differentcontexts originate from different meanings of the full verb dang. Though distinctfrom existent cross-linguistic studies, this conclusion demonstrates the features ofmodal verbs and future markers in Old Chinese.