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+ agoI 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.