Anyone who has learned English will wonder whether Koreans learning English make common errors and whether the errors are temporary or not. In this study, three types of errors were examined; word-order errors - inversion of the order fixed to Korean structure; the errors of co-occurring articles - juxtaposition of articles and other determiners; and overgeneralization - excessive use of wrong expressions. Five Korean adult immigrants in the U.S. took part in this study. The errors they made were analyzed through their writing samples they submitted. The findings showed that word-order errors decreased little by little, but overgeneralization increased in number. That was partly because the learners tried to apply what they learned in the class to their writing. Interestingly, the errors of co-occurring articles sharply decreased since the students memorized the rule that articles do not exist with other determiners in English. The errors the learners made went through a certain developmental process, which was on the continuum of interlanguage. The main concern of this study is interlanguage development of adult Korean learners of English, and it will cover EFL and ESL situation.