The purpose of the study is to identify the developmental stages in the acquisition of grammatical morphemes that Korean English learners undergo while living in English speaking countries. The study has proposed three hypotheses with three groups of children according to the duration of their exposure to English. The selected groups have been given a test to determine their understanding of grammatical morphemes after their exposure to English from a year to two years. Each group has demonstrated some features of acquiring grammatical morphemes. The results have provided that a certain set of grammatical morphemes are acquired according to the duration of their exposure to English. The study also has supported a conclusion that the proposed hypotheses have been proved, and there is an evidence that an order in acquisition of grammatical morphemes exists in L2.