The aim of my thesis is to analyse and describe the most common errors made by first year university students majoring in English. My other aim is to try to find reasons for the errors. The thesis contains two major parts: the first part introduces the Common European Framework, which serves as the standard for language proficiency. In the light of it, the assumed proficiency level of the students will be established and it will be compared to the assessment criteria of the English Yardstick Exam. The second part is the presentation of the findings of my own research, in which I analysed 50 argumentative essays in terms of grammar, vocabulary, communicative quality, and composition features. The commonest errors will be presented in order of occurrence with examples taken from student essays.