This study aims to investigate possible factors contributing to subject-verb agreement errors in Swedish junior and senior high school students' English written production. The sample data is collected from the Swedish Learner English Corpus (SLEC), which comprises student texts produced in a classroom setting. The texts are randomly chosen but evenly distributed in terms of binary gender, school year, and type of high school program. In this study, the texts included in the scope are written by students attending a Swedish-speaking school with Swedish as their first language. Errors are classified as overgeneralization or transfer and further classified in relation to the subject type, the verb type, and the distance between the subject and the verb. The classification of all correct instances of subject-verb agreement is also performed to further investigate possible error explanations. A total of 41 agreement errors were found in 24 texts written by students in the 9th and 11th grades. The results show that overgeneralization is more frequent than transfer errors. Overgeneralization suggests that the students are aware of the third-singular form but do not always apply it correctly, while transfer errors show a potential lack of awareness or attention to the form. In both cases, the errors indicate that these students have not automatized the principle yet. Errors are often related to subject types “a pronoun” or “a noun/noun phrase" and the verb be, which is the most frequently used verb. Most of the errors occur when the subject and the verb are in immediate contact, and more than half of them involve a relative pronoun as subject, which indicates that the learners have misinterpreted the grammatical principle or have not fully acquired it. Overuse of the third-person singular form can also be an effect of teaching and explicit learning, which makes learners apply the form whenever it seems possible and relevant.