11.13 since的四種用法
?
  1) since +過去一個時間點 (如具體的年、月、日期、鐘點、1980, last month, half past six)。
  I have been here since 1989.
 
  2) since +一段時間+ ago
  I have been here since five months ago.


  3) since +從句 
  Great changes have taken place since you left.
  Considerable time has elapsed since we have been here.


  4) It is +一段時間+ since從句
  It is two years since I became a postgraduate student.













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