In this paper, we examine one wh-construction in Singapore English, which signals a demand for justification, and show that there is a systematic correlation between its structural and pragmatic properties. We suggest that this wh-construction is based on the imperative, and inherits the structural properties associated with the relatively more polite version of the imperative. In Singapore English, this is the version that makes explicit mention of the second person subject, whereas in Standard English the use of you in the imperative decreases politeness. After a careful comparison of the pragmatics of the imperative in Standard English, Singapore English and Chinese, we conclude that the asymmetry between the why-construction in Standard English and in Singapore English can be accounted for by substrate influence from Chinese, from which the Singaporean construction has inherited its politeness constraints.