1) since +过去一个时间点 。
Ihavebeenheresince 1989.
2) since +一段时间+ ago
Ihavebeenheresincefivemonthsago.
3) since +从句
Greatchangeshavetakenplacesinceyouleft.
Considerabletimehaselapsedsincewehavebeenhere.
4) Itis +一段时间+ since从句
ItistwoyearssinceIbecameapostgraduatestudent.