Based on the Failed Functional Features Hypothesis (FFFH) (Hawkins and Chan, 1997), the current study aims at investigating L1 Thai learners? acquisition of the wh-operator movement in English open interrogatives. Two primary objectives included (1) to explore whether the wh-operator movement in English can be acquired by L1 Thai learners and (2) to account for the data by the FFFH. Due to the non-existence of the strong uninterpretable [uwh] feature in Thai, it is hypothesized that L1 Thai learners of English fail to acquire English open interrogatives by means of the operator movement and that variability in English open interrogatives is evident in both production and perception. A Question Formation Task (QFT) and A Grammaticality Judgement Task (GJT) were administered to 20 intermediate and 20 advanced L1 Thai learners. The findings revealed that, despite their low suppliance rates of resumptive pronouns in the QFT, both L2 groups persistently failed to detect the presence of resumptive pronouns in the GJT, with the rejection rates remarkably lower than those of the native controls. Moreover, their use and judgement of resumptive pronouns were rather unsystematic; that is, they seemed to produce or accept open interrogatives irrespective of the presence of resumptive pronouns, which may be ascribed to the fact that the Thai language allows the resumptive strategy in some structures such as the relative construction. In addition, an asymmetry between subject extraction and object extraction was found in the learner data, possibly owing to processing difficulties posed by the former. The study, therefore, concludes that L1 Thai learners cannot acquire the wh-operator movement in English, driven by the strong uninterpretable [uwh] feature absent in Thai, lending vigorous support to the FFFH and, thus, the proposal that Universal Grammar (UG) is partially available to adult L2 learners.