It’s……since…… 用的时态主句从句的分开讲

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.

it's 是 it has 明显是用现在完成时
since后边用的是一般过去时..

It’s+一段时间+since+某种完成时态

就这样了````
不是很难````

主句一般用现在完成时
从句一般用一般过去时

例：
It has been 2 years since I last met him.

it is +一段时间+since+过去时