Grammar is considered to be the most difficult part in Korean language learning. The complex grammar system of Korean Language offers many different linguistic devices to express various grammatical meanings. The morpheme "-ass-", indicating the past tense, isparticularly difficult to Chinese students and thus becomes one of the most important grammatical morphemes in the teaching of grammar. Based upon precedent research findings, this article makes a comparison between the past tense in Korean and that in Chinese. After analyzing the error models, the author explains why Chinese learners commit such errors, and proposes an operational teaching plan for the morpheme "-ass-".