The importance of explicit form-focused instruction is well recognized these days and the research has been focused on how to teach grammatical forms. Among several issues concerning instructional styles, this study deals with instructional modes: input-based and/or output-based instruction. There have been conflicting research findings regarding the efficacy of input-based or output-based instruction. To these conflicts the third view is added, which argues that input practice is better for comprehension skills ,and output practice for production skills, seeing language as one of many skills we can learn. Given this background, the present study proposed an experiment to test the argument of skill acquisition theory, and the results indicates that the specificity of practice effect in skill acquisition theory did not work for the learning of the third person singular -s of English.