1) since +过去一个时间点　。

　　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.