This paper investigates the grammatical phenomena that create difficulties in the second language learning of English for Swedish L1 students. By analyzing written English texts produced by Swedish as L1 students, an attempt is made to identify which grammatical errors are the most commonly occurring when Swedish students produce English texts. An attempt is also made to provide explanations for why these grammatical errors occur. The purpose of this study is to get an understanding of which errors are frequent in an L2 classroom and what causes the grammatical errors by taking into account the students’ bilingual context and second language development. To answer the research questions, both qualitative and quantitative research methods are used. The material consists of 20 authentic texts produced by 9th grade students from a Swedish elementary school. The outcome shows that most of the grammatical errors produced by the students arise due to negative transfer (interference) from the students’ L1, Swedish. The analysis of the grammatical errors was carried out using a contrastive analysis where the grammatical structures of Swedish and English were compared. The errors that the contrastive analysis could not explain, were explained using the following concepts: overgeneralization, processability theory, and communication strategies.