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.