it has been since.用法

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.

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)