This study revisited L2 morpheme development order among Korean young learners of English. Taking into account the spirit of the current English curriculum in South Korea, the current study examined L2 morpheme development from two different perspectives: accuracy and fluency aspects. In the field of second language development, many studies have claimed the L1 effect in the acquisition of English morphemes, opining that the absence or presence of the corresponding morphemes in L1 affects the advancement or delay of the L2 morphemes. However, L1 effect appeared to be morpheme-specific in these previous studies, which raised a question if L1 alone is truly a definitive factor to explain the variability in L2 morpheme development. Thus, semantic aspects of morphemes, specifically semantic interpretability at LF, was additionally considered as part of factors in order to explicate the interlanguage variability. The Interpretability Hypothesis and Levinson's Mapping problem inspired this model testing. Investigating the fluency aspect, the current study adopted a poststructuralist perspective in interpreting learners' interlanguage, in order to understand their language use as processes of meaning making and communication from a more "emic" approach. Wug test is widely used to measure first language learners' morphological metalinguistic knowledge, but has not been adopted much for EFL learners. As the current study focused on morphemes, the applicability of this test among EFL learners was also tested to discuss its appropriateness for EFL contexts. The present study revealed semantic interpretability affects learners' morpheme development even more than L1 effect alone does, and when interpretability was combined with L1 effect as an independent factor, it was found to explain morpheme developmental patterns considerably. The poststructuralist analyses uncovered many cases where linguistic strategies/tools were appropriated to negotiate with the gap between the learners' L2 linguistic knowledge and conventional use of L2. However, when quantitatively approached, even the poststructuralist perspective was found not to reduce the potential of misunderstanding caused by the incorrect morpheme uses by far. Wug test demonstrated a reliable predictability on how precisely EFL learners produced morphemes in the natural speech data. Based on these findings, the present study suggested some theoretical and pedagogical implications. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]