As a way of language output, writing reflects how much a language has been input.
Therefore, it is regarded as the most effective means to detect the input of the language.
In addition, the errors in learners' writing are solid evidence to reveal the problems in
present language teaching and learning. As Corder points out (quoted from Carl James
2001:12), errors are significant in three respects: they tell the teacher what needs to be
taught; they tell the researcher how students" learning proceeds; besides, they area means
whereby learners test their hypotheses about the Second Language (L2). Therefore, error