1) since +過去一個時間點　(如具體的年、月、日期、鐘點、1980, last month, half past six)。

　　I have been here since 1989.

　　2) since +一段時間+ ago

　　I have been here since five months ago.

　　3) since +從句

　　Great changes have taken place since you left.

　　Considerable time has elapsed since we have been here.

　　4) It is +一段時間+ since從句

　　It is two years since I became a postgraduate student.