This study is an investigation of Swedish junior high school students’ English grammar knowledge, based on their written production. The Swedish students’ written productions where analyzed using error analysis as a method to find the most frequent errors that the students make. The errors were identified in texts that were produced in two different ways (free and controlled), and these texts were then compared. The study was conducted through (a) identifying the most frequent grammatical errors in Swedish students’ written production, (b) analyzing what causes the errors to be made, and (c) distinguishing if there is any difference between free written or controlled written production. The students’ grammatical knowledge was discussed on the basis of the results of these procedures. The results show that the most frequent errors made by Swedish junior high school students are errors of verb tense, verb inflection and subject-verb agreement. The errors are probably caused by lack of grammatical knowledge, but also by incorrect transfer from Swedish into English. Furthermore, the results show that the students from the two target groups to some extent make the same type of errors regardless of their productions were free written or more controlled.