The study investigated variable production of English past tense morphology by an L1 Thai-speaking learner of English. Due to the absence of the past tense inflectional morphology in the Thai language, production of English past tense morphemes poses a persistent problem for L1 Thai-speaking learners of English. Hypotheses have been made in accordance with Lardiere's (2003) Missing Surface Inflection Hypothesis (MSIH), which predicts that the participant possesses the syntactic knowledge of the English past tense morphology. The participant's variability in L2 production of English past tense morphology was not due to a lack of L2 grammatical knowledge but rather the result of syntactic mapping to the morphophonological form i.e. extra-syntactic factors. The results confirmed the MSIH, suggesting that English past tense morphemes were acquired but the production problems lie in accessing the morphology.