［动词的时态］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)。例如：
I have been here since 1989. 1989起，我一直在这儿。
2) since +一段时间+ ago.例如：
I have been here since five months ago. 我在这儿，已经有五个月了。
3) since +从句。例如：
Great changes have taken place since you left. 你走后，变化可大了。
Great changes have taken place since we were here. 我们走后，变化可大了。
4) It is +一段时间+ since从句。例如：
It is two years since I became a postgraduate student. 我考上研究生有两年了。
