Korean is classified as an intonation language along with Japanese, English, French, and Arabic; the speaker can express different meanings with the same sentence using different intonation contours. Considering the correct utterance and communication of the learners, the importance of the function of intonation and teaching intonation cannot be overlooked. It is not easy for a foreign speaker to speak with correct intonation like native speakers or to learn the differences in the meaning only by hearing intonation. Therefore, learners may not notice the changes in meaning according to the differences in intonation because they only think of the morphological identity of a sentence pattern, unless the changes according to intonation are explicitly taught. This study began to emphasize the importance of teaching intonation, taking into consideration the communication of the learners. Among the many sentence patterns, which have different meanings according to intonation, this study focused on ‘-을걸.’ The material used in the tests and lectures in this study is semi-colloquial language from a television drama, which is language that is most similar to the utterances of colloquial Korean. This study confirmed the significance the effect of teaching intonation has with visible results through experimentation. The change in the percentages of correct answers for the tests before and after the teaching showed a significant statistical difference. Through the entire course of the experiments, the learners replied that they were reminded that the same sentence pattern could have different meanings with different intonation and that they could recognize that intonation could cause changes in the meaning. Consequently, this study is significant because it highlights again the importance of teaching intonation and the necessity to teach the learners as befits them.