since和for的区别:

since的用法

(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.

for的用法：

for+一段时间

I have lived here for three years.