This article analyzes two elements in Mandarin Chinese that denote repetition, you 'again' and zai 'again'. It has been assumed that you occurs in realis contexts, whereas zai occurs in irrealis contexts. We find that in fact each may occur in either context; furthermore, we argue that these two elements should be distinguished in terms of the structural positions they adjoin to and the event structures they occur in. We adopt Shen's (Journal of East Asian Linguistics 13: 309-336, 2004) framework of phrase structure and event structure for Mandarin Chinese sentences, and show that you adjoins to a dynamic AspP, and zai to a static vP. Our analysis accounts for a number of properties of you and zai, including their relative structural heights and their ability to induce change of the event structure of the predicate.