| Acknowledgements | 第1-7页 |
| Abstract | 第7-9页 |
| 摘要 | 第9-13页 |
| Abbreviations | 第13-14页 |
| List of Tables | 第14-15页 |
| List of Figures | 第15-16页 |
| Chapter One Introduction | 第16-20页 |
| ·Introduction | 第16-18页 |
| ·Research Purpose and Significance | 第18-19页 |
| ·Overview of the Thesis | 第19-20页 |
| Chapter Two Literature Review | 第20-35页 |
| ·Public Service Advertising:an Overview | 第20-26页 |
| ·The Definition of PSA | 第20-21页 |
| ·Classification of PSA | 第21-23页 |
| ·Sponsor Perspective | 第22页 |
| ·Medium Perspective | 第22-23页 |
| ·Subject and Content Perspective | 第23页 |
| ·Purpose Perspective | 第23页 |
| ·Characteristics of PSA | 第23-26页 |
| ·Human-Centeredness | 第24页 |
| ·Public Service | 第24-25页 |
| ·Non-Profit | 第25页 |
| ·Social Obligation | 第25页 |
| ·Thought-Provoking | 第25-26页 |
| ·Approaches to the Analysis of PSA | 第26-30页 |
| ·The Sociological Approach | 第26-27页 |
| ·The Semiotic Approach | 第27-28页 |
| ·The Linguistic Approach | 第28-29页 |
| ·The Functional Approach | 第29-30页 |
| ·Limitations of Previous Studies on PSA | 第30页 |
| ·Modality | 第30-35页 |
| ·Studies on Modality Abroad | 第30-32页 |
| ·Studies on Modality in China | 第32-33页 |
| ·Summary | 第33-35页 |
| Chapter Three Theoretical Foundation | 第35-47页 |
| ·Systemic-Functional Grammar:A Brief Introduction | 第35页 |
| ·Definitions of Modality | 第35-37页 |
| ·Functional Viewpoint on Modality | 第37-40页 |
| ·Linguistic Realizations of Modality | 第40-46页 |
| ·Finite Modal Operators | 第40-41页 |
| ·Modal Adjuncts | 第41-45页 |
| ·Mood adjuncts | 第41-44页 |
| ·Comment adjunct | 第44-45页 |
| ·Expansions of the Predicator | 第45页 |
| ·Modal Clauses(metaphors of modality) | 第45-46页 |
| ·Summary | 第46-47页 |
| Chapter Four Research Methodology | 第47-50页 |
| ·Data Collection | 第47-48页 |
| ·Data Analysis | 第48-49页 |
| ·Validity of the Research | 第49-50页 |
| Chapter Five Research Findings and Discussions | 第50-72页 |
| ·The Frequency of Modality Occurrence | 第50-52页 |
| ·Comparison of total number of modal expressions in two kinds of samples | 第50页 |
| ·Comparison of frequencies of modality occurrence in Chinese and American PSA texts | 第50-52页 |
| ·Summary | 第52页 |
| ·Comparison of different modal expressions between English andChinese | 第52-65页 |
| ·Comparison ofModalAdjunctbetween English and Chinese | 第52-56页 |
| ·Comparison ofFinite Modal Operator between English and Chinese | 第56-62页 |
| ·Comparison of Types of Finite Modal Operator | 第56-60页 |
| ·Comparison of the Value of Finite Modal Operators | 第60-62页 |
| ·Comparison of Expansions of the Predicator between English andChinese | 第62-63页 |
| ·Comparison of Metaphors of Modality between English and Chinese | 第63-64页 |
| ·Summary | 第64-65页 |
| ·Comparison of types and values attached to modality between Englishand Chinese samples | 第65-70页 |
| ·Types and values attached to modality in English samples | 第65-66页 |
| ·Types and values attached to modality in Chinese samples | 第66-67页 |
| ·The distribution of modality of probability and obligation andtheir corresponding value between English and Chinese | 第67-70页 |
| ·Value of Modality of Probability | 第67-68页 |
| ·Value of Modality of Obligation | 第68-70页 |
| ·Summary | 第70-72页 |
| Chapter Six Conclusion | 第72-74页 |
| ·Sum-up | 第72-73页 |
| ·Limitations and suggestions for further study | 第73-74页 |
| References | 第74-78页 |
| Appendix Ⅰ | 第78-84页 |
| Appendix Ⅱ | 第84-102页 |
| 攻读学位期间发表的学术论文 | 第102页 |
