印度東北部的Ao語呈現一些從類型學的角度來說很有意思的特徵，我們從這些特徵可以看到Ao語時標誌歷史發展的一些線索。作者認為Ao語原來有二元的語氣系統：實然（realis）和非實然（irrealis），前者無標誌，後者有標誌。Ao語把其中兩個名物化標誌的作用延伸到標記時的不同範疇，而原來的無標誌的實然範疇卻被歸納到標記時範疇的系統中，用來標記過去時範疇。非實然標誌雖然與這些後期語法化的時標誌體系上是對立的，但其功能仍然是標記獨立而不限於某種時段的語氣範疇。參看其他語言的現象，作者發現名物化標誌通常是時標誌語法化的普遍來源。

Ao demonstrates some typologically intriguing characteristics that shed light on the diachronic development of its tense marking morphology. The paper proposes that the language originally had a binary mood system contrasting a zero-marked realis with an overtly marked irrealis mood. It then extended the functions of two of its nominalizers to encoding temporal distinctions, and the old zero-marked realis mood became integrated into the newly grammaticalized system of temporal deixis as the zero-marked past tense category. The irrealis marker occurs in paradigmatic opposition with these newly grammaticalized tense markers, but continues to encode an independent category of mood that is not constrained to any particular temporal deictic setting. Drawing on additional evidence from other languages, it is demonstrated that nominalizing morphology turns out to be a rather commonly exploited source for the grammaticalization of tense markers.