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