11.13 since的四种用法
  1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
  I have been here since 1989.
  2) since +一段时间+ ago
  I have been here since five months ago.

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

  4) It is +一段时间+ since从句
  It is two years since I became a postgraduate student.