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)