It is 3 years since he joined the army.为什么用is不用has been

Question

It is 3 years since he joined the army.为什么用is不用has been

Answer 1

since是现在完成时的标志,但是since+过去的时间点或是过去时态从句,下面的例句是摘抄的,你参考下.我觉的,两个应该都行,你说的应该也是对的.因为joined 也是瞬时动词.
It is / has been + 一段时间 +since 从句.例如：
　　It is / has been three years since we saw each other last time.
　　我们上一次见面是在 3 年前.
　　It is four years since I became a student.
　　我上了 4 年学了.
　　（注意：状语从句中常用短暂性动词的一般过去时作谓语动词）
　　注意：since 从句里的谓语动词是瞬间动词时,表示这段时间从该动作开始时算起； since 从句里的谓语动词是延续性 / 表状态的动词时,则表示这段时间从动作结束时算起.例如：
　　It's five years since he lived here.
　　他离开这儿有 5 年了.（ live 为延续性动词）
　　It's ten years since he joined the army.
　　他参军已有 10 年了.（ join 为瞬间动词）