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.