The English past counterfactuality is difficult for non-native learners to learn and understand due to cross-language discrepancies. Oftentimes, individuals unavoidably develop the interlanguage and this language system becomes fossilized regardless of the amount of exposed time and input. The purpose of the study was twofold: (a) to examine 126 Thai EFL university students' production of the English past counterfactuals through the grammar translation method (GT); (b) to qualitatively investigate how interlanguage fossilization exerts an influence on the students' production. The results showed that their ability to use the English past counterfactuals was relatively low, whereas the errors of its structural elements could be found across the entire production. It was further observed from the interview that the low productive ability resulted from the L1 transfer and the training migration. The pedagogical implications for foreign language instructors are discussed.