1.1 Definition of SLA

A second language (L2)can be defined as any language that is learned after the acquisition of the mother tongue.In this context,learning of a second language can refer to the learning of a third,a fourth or even a fifth language.An L2 is commonly called a target language in a learning situation.But what does it mean by SLA,or what does an SLA researcher do in this field?Before we define this term,let us examine the following quoted definitions:

"Second language acquisition research"refers to studies which are designed to investigate questions about learners'use of their second language and the processes which underlie second language acquisition and use.

(Lightbown,1985)

SLA is thought of as a discipline devoted to discovering and characterizing how it is that a human being is able to learn a second language:what pre-knowledge does he or she bring to the task,what set of learning procedures does he or she use,what strategies are appropriate for certain phenomena and not others,etc.

(Schachter,1993)

(SLA) is concerned with what is acquired of a second language,what is not acquired of a second language,what the mechanisms are which bring that knowledge (or lack thereof) about and ultimately,an explanation of the process of acquisition in terms of both successes and failures.

(Gass,1993)

By SLA we mean the acquisition of a language after the native language has already become established in the individual.

(Ritchie&Bhatia,1996)

From the above definitions,it is easy for us to see a duel purpose in SLA:all definitions agree that the goals of SLA are to study,discover and characterize what is acquired of an L2 and how it is acquired.This book focuses on three elements.The first two are the same as the above:what and how.First,we focus on the developing knowledge and use of the L2 being learned.The scope of SLA includes informal L2 learning taking place in naturalistic contexts,formal L2 learning taking place in classrooms,and L2 learning that involves a mixture of these settings and circumstances.In addition to what is learned of an L2,we are equally interested in what is not learned in these contexts.Second,we focus on the study of how an L2 is learned.We are interested in how learners create a new language system with limited exposure to an L2,and how they make hypotheses about the rules of an L2.Considering how the rules are constructed in the learner's mind,the study of SLA is closely related to other areas of study such as linguistics,psychology,psycholinguistics,sociology,sociolinguistics,discourse analysis,conversational analysis,education,and so forth.Besides the questions of "what"and "how",our third concern in this book is "why":why some learners make more proficiency than others do.It is a fact that in any situation,there must be some learners who are more successful in learning an L2 than others.The research of "why" tries to answer this question.

This field of SLA research has both theoretical and practical importance.The theoretical importance is associated with our understanding of how language is represented in mind and whether or not there is a difference between the way language is acquired and processed and the way other kinds of information are acquired and processed.It is assumed that understanding how languages are learned will lead to more effective teaching practices.To speak broadly,a knowledge of SLA may help educational policy makers set more realistic goals for programs for both foreign language courses and the learning of the majority language by minority language children and adults (Spada&Lightbown,2008).