By comparing a corpus of Korean learners of English with a native speaker corpus, this study shows to what extent and in what ways Korean learners acquire English modal verbs can, could, may, and might (Hunston 2002). This corpus study revealed that the Korean learners underused could, may, and might. Two factors can explain the pattern of the Korean learners’ acquisition of the 4 modal verbs. First, the difficulty of combining tense with the modal verbs impedes the learners’ preterit form use of the modal verbs. Second, the epistemic modality is acquired much later than the root modality such as possibility, permission, and ability (Gibbs 1990). Since may and might typically associate with the epistemic modality, the learners rarely used those modal verbs.