since的四种用法
1） since +过去一个时间点　（如具体的年、月、日期、钟点、1980, last month, half past six）。
I have been here since 1989.
2） since +一段时间+ ago
I have been here since five months ago.
3） since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4） It is +一段时间+ since从句
It is two years since I became a postgraduate student.