The paper describes a case study in which the main objective is to understand how engineering students can improve their writing skills, regarding spelling and syntax, when taught specifically on these issues. The methodology Writing To Learn is applied in two courses and, making use of the written texts, the students' writing skills are assessed and evaluated. In one course, writing skills are taught and assessed and in the other they are only assessed. The comparison allows conclusions on the success of teaching writing skills, the influence of text styles and the differences between basic and advanced writing skills. It was found that writing skills were successfully taught, particularly with regard to basic writing skills. Advanced writing errors are twice as common as basic writing errors. Schematic writing styles favour a reduced number of writing errors. (Contains 8 tables and 6 figures.)