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.(跟一般过去时)
Considerable time has elapsed since we have been here.(现在完成时)
4、It is +一段时间+ since从句
It is two years since I became a postgraduate student.(跟一般过去时)