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