This study investigates Mandarin-speaking children's knowledge of event semantics in interpreting spatial modifiers with "zai" 'at' after a posture verb or before a placement verb. The event-semantic principles investigated include subevent modification (Parsons, 1990) and aspect shift (Fong, 1997). We conducted an experimental study using modified forced choice, video choice, and elicited production techniques with five groups of children (two- to six-year-olds) and an adult control group. Three-year-olds were sensitive to the ambiguity of "zai"-PPs with placement verbs and posture verbs, suggesting guidance from principles of aspect shift and subevent modification. On the other hand, distributional properties of the input play a role in acquiring the interpretation and word order of "zai": e.g., four-year-olds significantly differed from adults in accepting non-target V-"zai" sentences, as some verb classes can take postverbal prepositional phrases with "zai" while others cannot in adult usage.