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.