It is three years since he joined the army.为什么是用joined而不是用ha

It is three years since he joined the army.为什么是用joined而不是用ha It is 3 years since he joined the army.为什么用is不用has been 用since的话,本句应该是：It is ten years since he joined the army. It is three years since I joined the army.改成完成时态 I It was three years ago that he joined the army与It is three y it is four years since I joined the army.这里的is,为什么不用was呢? It __four years since i joined the army.这里的is,为什么不用was呢? t is three years since he joined the League?it is three year John joined the army three years ago.(用since或for改写） 怎么写. Three years ____ ____ since he joined the army 填have passed 看到一句It is 3 years since he joined the army.和一句It has been th it has been 5 years since he joined the army 是表肯定还是否定呢 求教英语大