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.