Learning a new language equals learning new grammatical skills, which takes a lot of effort and time. Therefore teachers need to maximize the effects of teaching grammar on their students’ language acquisition. There are several theories on how this might be done: This paper will present a study of the effect of giving feedback on students’ essays by means of underlining. The two main aims of this paper were to find out whether or not students will improve in their writing skills when given feedback by means of underlining and if there is a difference between male and female students in their improvement. The material used in the study consists of three essays each from fifteen students (seven male and eight female students), collected at the beginning, in the middle and at the end of their first semester in upper secondary school. The results of the investigation show that the students do improve in their writing skills when given feedback by means of the underlining method. The investigation also shows that the male and the female students showed equal improvement in their grammar skills. Keywords: grammar in L2 students’ writing, tense, plural, genitive, feedback methods.