since的四种用法_语法技巧

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.