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.