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.