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.

来源:爱思英语网-英语三级考试