The intended purpose of this essay was to examine student essays to find out if there are any first language interferences in Swedish students’ written English. First language interference has been researched before, which will be presented in the essay, amongst other terms such as translanguaging, acquisition and language transfer, all explained in the theoretical concepts section. The material for this contrastive analysis was collected from a gymnasium in the south of Sweden. A total number of eighteen essays were collected and examined, with a total number of 235 interference errors, with prepositions being the largest number, and 735 non-interference errors, with spelling topping with 305 errors. In conclusion, the investigation showed some interference errors, but also highlighted the non-interference errors of the students’ essays. Finally, the study present examples for future research in the field, as well as examples of how teachers can work with their students to prevent first language interference errors and make their students aware of the influence their mother tongue have when learning a second, or third, language.