【Abstract】This paper reports on a study which investigated the effects of correction of students’ grammatical mistakes on acquisition. Specifically， it compared the effects of when to correct， immediate vs. delayed correction， and how to correct， explicit vs. implicit correction. It also investigated the relative effects of correction of morphological vs.syntactic features and correction of developmental early vs. developmental late features.Data for the study were collected from 50 intermediate level students of English as a foreign language in Dalian University of Finance and Economy. Each participant was required to read and then retell a written text in their own words during an oral interview with the researcher. During or following the interview， the researcher corrected the participants’ grammatical mistakes implicitly or explicitly. Individualized tests focusing on the mistakes that had been corrected were constructed for each participant and administered. Statistical analyses were conducted on the scores participants received on their individualized tests.
　　【Key words】mistake correction； acquisition； pedagogy； morphological； syntactic
　　1. Introduction
　　Mistake correction has been widely recognized as vital in second language acquisition and teaching in the last two decades.
　　According to the error analysis hypothesis， students’ production of erroneous structures can assist us in understanding their cognitive processes. More specifically， mistakes are evidence of the processes and strategies of language acquisition. Learners are assumed to create a language system known as inter-language， in which learners themselves impose structure on the available linguistic data，， drawing upon their knowledge of the mother tongue and of the second foreign language. Inter-language refers to independent systematic knowledge of a second language native language children as well as second language learners hold in the process of learning. Learners formulate an internalized system which enables them to synthesize linguistic data， the system being distinct from both the learner’s native language and the target language.
　　2. Form-Focused Instruction
　　Focus on Forms and focus on Form
　　Form-focused instruction can further be divided into two types， distinguished from each other nominally only by singular vs.plural terms： focus on Forms and focus on Form. The focus on Forms approach involves teaching grammar in isolated linguistic forms in accordance with a structural syllabus. Here the teacher preselects specific forms for attention， and meaning and communication play a lesser role in this type of instruction. In other words， it is not a meaning -centered approach. And it is believed that focus on Forms cannot help learners acquire structures and that it hinders language learning.