The thesis consists of four chapters. Chapter one is the introduction part, which introduces the object and meaning of the research and the structure of the thesis. Chapter two is about previous studies on chunks, which introduces the definition and classification of chunks and studies on chunks. Chapter three illustrates the design of research. It covers research questions, data collection and data analysis. Chapter four is about results and discussion, which is composed of three parts—distribution of different lexical chunks, persity of lexical chunks and mistakes in use of lexical chunks in English majors’ argumentative writings.
2    Previous Studies on Lexical Chunks
2.1 Definition and Classification of Lexical Chunks
Literature on lexical chunks indicates that scholars have pided opinions of terms and core concepts of lexical chunks and also of definition, identification and classification of lexical chunks. According to the lexical approach of Michael Lewis(1993), lexical chunks are fixed or semi-fixed language structures that have both syntactic and lexical characteristics. From the point of view of Ma Guanghui(2009), the lexical chunk, as one form of multi-word units or constructions, can be defined as the smallest semantic structural unit which is composed of multiple words and can be used independently to produce sentences or utterances and can realize certain grammatical discourse or pragmatic functions.

Nattinger(1992) pides lexical chunks into four categories from the point of structures and functions: polywords, institutionalized expressions, phrasal constraints and sentence builders. Polywords are phrases which have similar functions to words, like by the way, so long. Institutionalized expressions are idioms, proverbs and formulistic expressions, like how are you, once upon a time. Phrasal constraints refer to structures which are open partially, such as a_ ago, see you_. Sentence builders are those providing the framework for the whole sentence, such as I think that, let me start with.

Biber(1999), according to the analysis of texts, classifies lexical chunks into 12 types by structure: (1) noun phrases+ of phrase fragments, (2) noun phrases+ other postpositive attributive fragments, (3) preposition phrases+ of phrase fragments, (4) other preposition phrase fragments, (5) It+ verb phrase/ adjective phrase fragments, (6) passive verbs+ preposition phrase fragments, (7) Be+ noun phrase/ adjective phrase fragments, (8) (verb phrases+) that clause fragments, (9) (verb/ adjective+) to phrase fragments, (10) adverbial clause fragments, (11) pronoun/ noun phrases+ be(+), (12) other expressions.

Xu Xianwen(2010) comprehensively analyzes definitions and classifications made in previous literature and practical analysis of texts and extracts target lexical chunks from texts according to structures. He pides lexical chunks into seven categories: (1) verb chunks, such as have a look at, find out, make up, (2) noun chunks, like a kind of, a few of, boys and girls, (3) adjective chunks, like formal and informal, physical and mental, (4) adverb chunks, like again and again, back and forth, (5) preposition chunks, such as because of, in such way, (6) conjunction chunks, like as long as, no matter, (7) clause chunks, such as it is true that, I think that. The clause chunks are the same as sentence builders put forward by Nattinger(1992) and clause fragments proposed by Biber(1999).
2.2 Studies on Lexical Chunks
Lexical chunks are language structures between words and sentences and have both the characteristics of words and sentences. Theoretically, scholars led by Nattinger and DeCarrico(1992) share a common consensus on lexical chunks— lexical chunks are combinations of words which can be standard or nonstandard, complete or incomplete, but they are all of practical values. In addition, lexical chunks have characteristics of psychological reality. They are stored in people’s memory or closely related to each other as a whole, so they are easily recognized and used in order to improve the speed of reading and fluency of expressing(Ma Guanghui, 2009:54-60). Lexical chunks, as larger linguistic units, combine grammatical rules, meanings and concrete contexts together. They are frequently used and of more stable structures—all these advantages have made chunks ideal units in English learning and teaching. 英语专业学生议论文写作中词块类型的调查研究(2):http://www.751com.cn/yingyu/lunwen_13227.html