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Chapter 1 Introduction

1.1 Research background of this dissertation 
English  modal  verbs  are  a  complex  semantic  system.  There  exists  gradience, ambiguity  and  merger  in  their  semantic  meaning.  Semantic  meaning  of  modal  verbs  is very sensitive to context. Different contextual factors have different influences on modal verbs. Consequently English modal verbs cause difficulties to natural language processing (NLP).  NLP  as  a  Science  processes  mountains  of  authentic  language  by  resorting  to computer.  Only  if  semantic  meaning  of  English  modal  verbs  is  correctly  dealt  with  can English  modal  verbs  be  conducive  to  the  study  of  NLP.  In  other  words,  Word  Sense Disambiguation (WSD) is a prerequisite for NLP. Time,  aspect  and  voice  which  fall into  important  grammatical  categories,  constitute linguistic context  of  modal  verbs.  When  Tang  and  Yang  (2008)  research  on  English modality from multi-perspective, they point out that every factor which constitutes context influences  on  semantic  meaning  and  context  has  restriction  on  generation  and understanding of discourse. Peng (2007) discusses interaction between modals in modern Chinese  expression  and  situation  and  concludes  that  situational  features  of  verbs  or sentences  can  tell  meanings  of  the  poly-semantic  modals  apart.  Liu  and  Peng  (2010) consider  the  interactive  restriction  when  Chinese  modality  and  aspect  appear simultaneously. It enlightens the author that grammatical categories like tense, aspect and voice  can  be  combined  to  study  English  modal  verbs.  English  modal  verbs  hardly co-occur with time reference. Hence time including three kinds of implied time: past time, present time and future time is used. Seeing that time is accompanied by aspect or voice, therefore time, aspect and voice are combined to be studied.  
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1.2 Objectives of this dissertation 
Questions that how do time, aspect and voice influence semantic meaning of modal verbs and how much do they influence semantic meaning of modal verbs have been tough problems  in  linguistics  which  urgently  need  to  be  addressed.  This  thesis  is  based  on  a corpus  of  real  language  with  about  2.1  million  data.  Modal  verb  must  as  target  word appears  nearly  one  thousand  times  in  the  corpus.  According  to  four  authoritative dictionaries,  modal  sense  of  must  is  divided  into  four  types  and  different &nb本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。