The present study aimed to investigate and analyze the most frequent grammatical errors in both oral and written production, what causes the errors as well as the possible correlation that may exist between them by means of a quantitative and qualitative method.The sample selected to perform the investigation is composed of 23 students who study the eighth course in the primary Swedish school. The data analyzed are obtained by two corpus composed of 23 written texts and 8 audios.The process followed throughout the investigation consists of a deductive method applying the Error Analysis theory. The grammatical categories are divided into verbs, prepositions, articles, gender and number concordance. In addition, in order to get a deeper understanding of the results, global, local, coherent and idiosyncratic errors are investigated.The results show that the errors in the written production are more frequent than in the oral production and that the error percentages in each grammatical category differs, predominating the errors in verbs in both corpus as well as the local and idiosyncratic errors.