Question

it has been since.......用法

Answer 1

It has been since +从句，从句用一般现在时。相当于It is +段时间 since+从句。
It has been ten years since he came here.
I t is ten years since he came here.

Answer 2

It is + 段时间 + since... / It has been + 段时间 + since...表示“从……起已有多长时间了”。两句型意思相同， 前者是规范说法， 后者是口语说法。
用法：since从句中常用一般过去时，若从句中是持续性动词的一般过去时， 则表示该动作结束有多长时间了。如：
It is three years since he smoked. 他不吸烟已有3年了。
试比较：
It is three years since he began to smoke.
t is three years since he has smoked. 两句均意为：他吸烟已有3年了。

注意：在这种结构中， since引导的从句绝对不能用否定式的谓语动词来表示否定意义， 试比较：
他好久没学汉语了。
误：It is a long time since be didn’t study Chinese.
正：It’s a long time since he studied Chinese.
(studied为持续性动词， 应从其动作结束时算起)