1) since +過(guò)去一個(gè)時(shí)間點(diǎn)　。

　　I have been here since 1989.

　　2) since +一段時(shí)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(shí)間+ since從句

　　It is two years since I became a postgraduate student.