The translation of English academic textbooks into Indonesian is pivotal in bridging language barriers and facilitating knowledge transfer in Indonesia’s higher education context. However, inaccuracies and unnaturalness in translations often complicate the readability and comprehension of these texts, posing challenges for educators and students. This study investigates the causes of unnatural translation choices in English-to-Indonesian academic textbooks, with a primary focus on nominal groups within the framework of Systemic Functional Linguistics (SFL). While the nominal group is the principal unit of analysis, the investigation necessarily considers its realisation within the clause structure. This includes an examination of Processes, Participants, and Circumstances to identify how nominal groups are realised and how their misrepresentation contributes to unnatural translation. The study employs a text analysis methodology, drawing on two widely used English-to-Indonesian academic textbook translations in language learning and psychology courses. The analysis focuses on how translation choices at the nominal group level affect all four metafunctions. The findings reveal that typological differences, particularly in the representation of nominal groups, influence not only the naturalness but also the accuracy of translations. Inaccuracies predominantly affect the experiential metafunction, on and, to a lesser extent, the textual metafunction, while unnatural translation issues extend across multiple metafunctions, including experiential, interpersonal, textual, and logical. Key issues such as mismatched collocations, redundancy, inappropriate textual references, informality and nominal group construction are identified as primary contributors to unnaturalness in translations. Additionally, multilingualism, oral language culture, and pronoun usage for inanimate objects may cause these issues. The misinterpretation of specialised technical terms further contributes to translation inaccuracies. The study offers insights into the analysis of unnatural translation through an SFL lens, contributing to translation studies and offering practical strategies for improving academic resources. These findings hold practical implications for translators and educators, supporting the development of more effective and accessible academic texts in Indonesia.