since的四種用法
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.
