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