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.
