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）.例如：
I have been here since 1989.1989起,我一直在這兒.
2） since +一段時(shí)間+ ago.例如：
I have been here since five months ago.我在這兒,已經(jīng)有五個(gè)月了.
3） since +從句.例如：
Great changes have taken place since you left.你走后,變化可大了.
Great changes have taken place since we were here.我們走后,變化可大了.
4） It is +一段時(shí)間+ since從句.例如：
It is two years since I became a postgraduate student.我考上研究生有兩年了.