6Krashen D. Principles and Practices in Second Language Acquisition [M]. New York.. Prentice- Hall, 1987.
7Long M. Focus on form: A Design Feature in Lan- guage Teaching Methodology [A].In K DeBot, R Ginsberg, C Kramsch (eds.). Foreign Language Research in Crosscultural Perspective [C]. Amsterdam: John Benjamins, 1991. 39--52.
8Nunan D. Understanding Language Classroom[M]. New York: Cambridge University Press, 1989.
9Ellis R. Understanding Second Language Acquisition[M]. Oxford: Oxford University Press, 1995.
10Skehan P. Second Language Acquisition Research and Task-based Instruction[A]. In J Willis, D Wills (eds.). Challenge & Changes in Language Teaching [C]. Heineman, 1996.