The aim of the thesis was to investigate the role of the monitor, or more accurately the role of explicit knowledge of the TL in correcting errors generated by the acquired knowledge system in written language production of university level L1 Hungarian learners of L2 English. From this perspective I carried out a small-scale research analyzing argumentative essays randomly selected from the 2011 English Yardstick Examination. The theory underlying my investigation was Krashen’s Monitor Theory, more specifically his Learning-Acquisition and Monitor Hypotheses. In order to investigate the role of explicit knowledge over acquired knowledge I looked at the corrections the students made during the writing of their compositions and examined to what extent the self-corrections resulted in the successful correction of errors.