（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.