This study deals with Korean learners' acquisition of English negatives. According to previous research, learners of English pass through a stage that demonstrates the characteristics of learners' interlanguage. Using a learner corpus (KELC), we show that Korean learners progress through several developmental stages before they master English negation. In addition, the errors made at each developmental stage reflect learners' knowledge of syntactic representation in terms of functional categories. Specifically, at the beginner level, the learners do not use auxiliaries at all. As they move to the intermediate level at which the functional category begins to be used, they start using auxiliaries in front of the negator. However, their outcomes are undermined with inflection errors either on the auxiliary or on the thematic verb. Finally, at the advanced level, the inflection errors disappear although still the present tense dominates where the past tense is required. This developmental pattern is in accord with the stages reported in the literature and the maturation of syntactic representation (i.e., functional categories).