since的四種用法
　　
1) since +過(guò)去一個(gè)時(shí)間點(diǎn)　(如具體的年、月、日期、鐘點(diǎn)、1980, last month, half past six)。
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.