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