for加一段时间 表示一个时间段 比如 for 4 years
就是4年整
所以句中的动词也应该是延续性动词
since 的用法比较复杂
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. 从1989年至今
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.
