现在完成时since的三种用法

现在完成时since的三种用法：与since +时间(起点)连用，表示自…以来。与since +一段时间+ ago连用，表示自…以前就…。与since +从句(从句用一般过去时)连用，表示自某事发生后就一直…。

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.