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. （在1989年我来到了这里）
2、since +一段时间+ ago
I have been here since five months ago. （自从五个月前,我已经在这里了）
3、since +从句
Great changes have taken place since you left.（自从你离开后，发生了很大的变化）
Considerable time has elapsed since we have been here.（我们到这里已经过了很长时间了）
4、It is +一段时间+ since从
It is two years since I became a postgraduate student.
