Loto, Praiwan. 2018. “The Honorific System in German as a Phenomenon of Courtesy: The Culturally Contrastive Analysis of Foreign Language Learning Exemplified through the Use of the Personal Pronouns Du and Sie by Thai German-learners at Chiang Mai University”. Journal of Letters 47 (3):339-78. https://so03.tci-thaijo.org/index.php/jletters/article/view/158850.