看到一句It is 3 years since he joined the army.和一句It has been th

看到一句It is 3 years since he joined the army.和一句It has been th 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 has been 5 years since he joined the army 是表肯定还是否定呢 求教英语大 It is three years since he joined the army.为什么是用joined而不是用ha since,the,is,he,ten,Party,joined,years,it (连词成句) he ___for three years.A has joined the army B has been in th It is said _______ he has joined the army. He has joined the army for ten years.和 He has been in the ar 15、 It _____ ten years since he left the army .A.is B.has C. It _____ ten years since he left the army . 　A. is B. has C. it is four years since I joined the army.这里的is,为什么不用was呢?