The Interface Hypothesis proposes that the pragmatic-discursive interface with syntax is more vulnerable to crosslinguistic influence than the syntactic-semantic interface [Tsimpli, Ianthi, and Antonella Sorace. 2006. "Differentiating Interfaces: L2 Performance in Syntax- Semantics and Syntax-Discourse Phenomena." In Proceedings of the 30th Annual Boston Conference on Language Development, edited by D. Bamman, T. Magnitskaia, and C. Zaller, 653-664. Somerville: Cascadilla Press]. The present study tests this hypothesis by analyzing the contrast between preverbal and postverbal subject position with unergative and unaccusative verbs in the Spanish of heritage language (HL) speakers. The results of a grammaticality judgment task and a short-answer task suggest that the syntactic-semantic interface is as vulnerable to the influence of English as the pragmatic-discursive interface for Spanish HL speakers. This inquiry also considers how formal instruction targeting the flexibility of word order in Spanish and its effect on the discursive context might impact the acquisition of subject position distinction with these verbs among HL learners of Spanish. Based upon the negligible effects of a pedagogical intervention conducted in an intermediate-level Spanish HL course in which the present participants were enrolled, both the syntactic-semantic interface and the pragmatic-discursive interface appeared impervious to formal instruction targeting flexible word order.