This research study investigated the stress patterns and metalinguistic knowledge of Thai learners on the pronunciation of English derivational suffixed words. The study compared how the learners performed before and after the praxis intervention, where the stress placement rules of English suffixed words were explicitly taught and trained. Thirty first-year university students were selected by stratified random sampling based on their proficiency levels and were grouped into the high and low proficiency groups. They took the pre-test which was the read aloud task of the base and the suffixed words. Their performance was recorded, and the stress patterns were analysed. Then, they joined the three-week praxis intervention sessions which included the video lessons, classroom activities, and homework assignments. Within a week after the praxis intervention, the participants took the post-test by performing the read-aloud task which included another list of the base and the suffixed words, and then they did metalinguistic knowledge elicitation task. The participants were further interviewed after the post-test. They reflected on their learning performance and provided their metalinguistic knowledge regarding stress placement of English suffixed words. The results of the stress patterns revealed that the high group performed the patterns which were in agreement with the English accentual system more often than the low group. The stress error patterns performed by the Thai learners revealed the intralingual and interlingual influences. The results of learners’ performance, before and after praxis intervention, showed that level of achievement in their learning was dependent on many factors such as motivation, time of exposure, and time practicing the rules and pronunciation. The results from the metalinguistic knowledge elicitation task revealed different metalinguistic knowledge between the high and low proficiency groups. The highly proficient learners used morphophonologically oriented knowledge more than the less proficient learners who used either morphologically or phonologically oriented knowledge more. The low proficient learners also gave numerous impressionistic answers. The findings exhibited that teaching materials and praxis intervention can help enhance metalinguistic knowledge and contribute to English language teaching and learning. The study also provides pedagogical implications for English language classrooms regarding English phonology and English pronunciation.