本研究檢視中古蒙古語至現代蒙古語時間語法系統的變化，並探討變化的動因、發展脈絡與方向。本文顯示蒙古語正經歷「分析－合成」的循環變化。一方面，中古蒙古語以動詞「過去時」形式，而現代蒙古語以動詞「完整貌」形式加上「非過去時」形式的存在動詞表示過去時，是蒙古語由合成性變得更具分析性的變化。另一方面，現代蒙古語的動詞「非過去時」詞綴-na/-ne的形成則是蒙古語由分析性變得更具合成性的例證。對稱性與經濟性是蒙古語動詞時間語法系統變化的動因。

The paper traces the development of the Mongolian temporal system from Middle Mongolian to Modern Mongolian, and deals with questions of how and why languages change typologically. It is shown that Mongolian is undergoing analytic-synthetic cyclic changes. On the one hand, Mongolian is getting more analytic, encoding past time with a combination of a perfective verbal noun and the present tensed existential verb. Previously it employed a synthetic inflectional verb to denote past tense. On the other hand, the emergence of modern non-past suffixes -na/-ne shows that there is also a series of changes making Mongolian more synthetic. Symmetry and economy are two opposing motivations for changes in Mongolian verbal paradigms.