This study examines the "ba" and "bei" constructions in Mandarin using data from the Tong corpus, a new multimedia longitudinal child language corpus. A unified aspectual account of the two constructions is proposed: both require telic predicates, and should thus correlate with the perfective rather than imperfective aspect for learners. Analysis of corpus and diary data reveals that Tong is generally sensitive to the telic requirement when he begins to use the two constructions around 2;0. His "ba" and "bei" sentences occur far more frequently in perfective than imperfective aspect, consistent with the Aspect Hypothesis. However, while the majority of the child's "bei" passives are perfective, the majority of his "ba" sentences are produced without overt aspect markers, among which most are irrealis (imperatives, modal sentences, etc.). The difference mirrors the pattern in adult input. These findings are corroborated by additional child Mandarin corpora. The acquisition of the aspectual properties of the "ba" and "bei" constructions is influenced by inherent knowledge, input and language-specific features of Mandarin, consistent with the multi-factorial account proposed by earlier studies.