The purpose of this research is to investigate the variable production of English Object-Experiencer (OE) psych predicates among 80 L1 Thai EFL first-year university students. Psych verbs are notable for posing difficulty for learners due to a unique property that violates Grimshaw's (1990) Thematic Hierarchy. The participants were given a cloze test and a translation test with six psych verbs and corresponding predicates, and six individuals were chosen to perform the spontaneous oral translation. Based on the Missing Surface Inflection Hypothesis (MSIH), the findings validated two of the study's research questions. First, variable production was detected in L1 Thai students' usage of OE psych predicates in three grammatical constructions examined. Second, the MSIH attributed variable production of L2 functional morphology to mapping problems: L1 Thai students were confused about how to link thematic roles to subject and object positions since the OE psych predicates breached the Thematic Hierarchy. Furthermore, the MSIH reveals that phono-syntactic factors play a role in variable production, indicating that during live translation, participants struggled with the -ed sound while forming correct copular verbs required in sentences with OE psych predicates.