In the process of language study and exploration,human beings’countless successful experience as well as lessons of failure has furthered the understanding of language step by step.An insight into previous research and problems will be likely to help contemporaries know what has been done.The In the process of language study and exploration, human beings’countless successful experience as well as lessons of failure has furthered the understanding of language step by step. An insight into previous research and problems will be likely likely help contemporaries know what has been done. The