This article adopts an input perspective in examining a poverty-of-the stimulus (POS) learning situation in second language acquisition (SLA). Analysis of grammaticality judgement data from 81 English-speaking and 85 Chinese-speaking learners of Japanese isolates triggering input that informed English learners of subtle semantic properties of the ni direct passive underdetermined by second language (L2) input. The study shows a sufficient correlation in the case of English learners between acquisition of the ni direct passive's triggering properties (available through input) and acquisition of its POS properties (unavailable through input). Importantly, those properties are direct consequences of affectivity, an underlying semantic property of the ni direct passive. That correlation does not obtain in the case of Chinese learners due to a positive first language (L1) effect. Additional corroborating evidence comes from acquisition of another Japanese passive, the ni yotte, for which no correlation was found between its non-triggering and non-POS properties for either English or Chinese learners as those properties are available through input. The article proposes that English learners' computation of a target-like conceptual representation of the triggering input leads to the restructuring of their lexical-conceptual representation of the ni direct passive. (Contains 5 tables, 2 figures, and 23 footnotes.)