苏州韦博讲解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.
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