t is three years since he joined the League?it is three year

t is three years since he joined the League?it is three year It is three years since he joined the army.为什么是用joined而不是用ha It was three years ago that he joined the army与It is three y It is three years since he smoked. It is three years since I joined the army.改成完成时态 I Gao Fang joined the League since three years ago(改为同义句） Gao It __is_________(be) three years since Lily joined the Green It is three years since the war broke out. 用since的话,本句应该是：It is ten years since he joined the army. he has been a soldier for three years.改为同义句：It is three year It is 3 years since he joined the army.为什么用is不用has been 求It is three years since he worked in his office.解释下since