This study aims to (1) describe the types of language errors in the explanatory text essays of class VIII students of Muhammadiyah 1 Kartasura Junior High School, (2) describe the forms of Indonesian language errors in student explanatory text essays, and (3) describe the accuracy of using Indonesian in student explanatory text essays. This is descriptive qualitative research focused on analyzing errors in Indonesian in the fields of phonology, morphology, and syntax. The research steps were carried out by collecting data on Indonesian language errors in the form of words, sentences, and paragraphs from the explanatory texts made by students, identifying errors in the explanatory texts made by class VIII students, grouping them into types of language errors, sorting the errors based on the frequency, and describe the mistakes based on the causes of the errors. The results showed 50 errors in Indonesian in the fields of phonology, morphology and syntax, consisting of 33 phonological errors, 14 morphological errors and 3 syntax errors. This study shows that class VIII students of SMP Muhammadiyah 1 Kartasura need to improve their understanding of writing an essay properly and correctly in accordance with the Indonesian Refined Spelling Guidelines (EYD).