This dissertation re-examines a puzzling restriction on assertability of certain zero-marked sentences in the grammar of Mandarin Chinese, the so called incompleteness phenomenon (Kong 1994; He 1994; Tang and Lee 2000; Gu 2007; Tsai 2008; Guo 2015; Sybesma 2019; Tang 2022, among others). Unlike the majority of the existing analyses which attribute incompleteness to some context-free grammatical requirement, I establish the novel generalization that incompleteness is sensitive to the explicit or contextually implied Questions Under Discussion (van Kuppevelt 1995; Ginzburg 1996; Roberts 1996/2012; Farkas and Bruce 2010), which straightforwardly captures a wide range of apparently heterogeneous conditions that render incompleteness absent in an explanatory way. Moreover, I relate incompleteness to a set of similarly constrained zero-marked forms in other languages, showing that those zero-marked forms in Mandarin can be syntactically and semantically well-formed. The dissertation provides a first formal pragmatic account of incompleteness, which attributes the two subcases of incompleteness discussed here, temporal incompleteness and degree incompleteness, to different pragmatic mechanisms. For the temporal case, incompleteness arises because of two incompatible R-based and Q-based implicatures (Grice 1967; Horn 1984). For the degree case, incompleteness arises due to a lexical pre- supposition encoded by the zero-marked sentences. The pragmatic account explains the QUD-sensitivity of incompleteness as well as captures native speakers’ intuition towards an incomplete sentence that it is ‘unfinished’ instead of outright grammatical. Under the current analysis, the degradedness is attributed to the failure of a context to avoid the conflict of implicature or to satisfy the presupposition, which can be potentially salvaged when the utterance is continued due to the dynamic nature of the context. The dissertation not only makes a variety of empirical and theoretical contributions to the incompleteness phenomenon, but also shows another effective cross-linguistic implementation of the notion of Question Under Discussion and the related tools in formal discourse theories including alternatives, implicatures, and presuppositions.