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.。