The research aimed to investigate the common grammatical in Thai to English translation of 34 students enrolled in the second semester of 2022 at a Thai Rajabaht University. The source of the 990 errors-corpus was taken from three different sample tasks from in-class assignments, including one fictional text, one informative text, and one cultural text. Data were analyzed by using content analysis, sorted by frequency and percentage. The investigation emphasized the sources' descriptions and the causes of errors rooted in similarities and differences between both language structures. Findings revealed that the most commonly found and notable errors ranged from countability (16.57 %), tenses (14.24 %), articles/determiners (13.13 %), fragment (10.10%), ellipsis (8.59 %), punctuation (8.38 %), respectively—the sources and causes of the errors derived mainly from the two languages' differences in grammatical systems by the influence of L1. The awareness of the differences was the key in promoting translation learning engagement, recognition, and production of appropriate translation from a source language to a target language. Additionally, the knowledge of both grammatical differences of the languages eventually benefited to translation instruction.