This paper will explore the difference between that as a complementizer and that as a relative pronoun. Most grammar books are self-sufficient in their explanation of the two clause types introduced by the same word: "One is a noun clause, and the other is an adjective clause." And when it comes to sentences of extremely similar outlook, they keep silent and let the students puzzle over them or give them up. I find it necessary to illustrate the distinction by all means. Generative transformation has aided me a lot in making the illustration. My standing as a Chinese also urged me to contrast the two languages in terms of this subject matter. What I am eager to find is not identical sentence patterns in both languages. To me, it is more important to see if the phenomena found in one language can help explain those found in another. I also want to know the role of interference in language acquisition after a contrastive study of this kind. This paper is divided into three sections. The first and the second deal respectively with that as a complementizer and that as a relative pronoun. A comparison will be found in the lines of the second section. The last section will talk about the two clause types in Chinese, leading up to a translation practice done by a number of Chinese students. And from the statistics, we might be able to see how interfering the two languages are to each other. By doing this, I have a pragmatic purpose in mind: What and how should a language teacher teach?