It has been years since I returned home,为什么since用,而不用when?分不清引导词该怎么用,请赐教.

It was 85 years还是 it has been 85 years? It has been 4 years since I smoked.怎么翻译? It has been 3 years since he came here. It has been there since a t_ years ago . it is 3 years since Tom has been abroad It has been five years since he has left for beijing这句话对不对啊 The tall tree ______ there for many years but it has been cut down.为什么是was 而不是has been It has been 5 years since this party----.如题 It has been 5 years since this party----.用came to power 为什么不能用 in power it is two years since I came here还是it has been two years since I came here 翻译 It's five years since he smoked It has been 2 years since we worked here Our school is a new one,it _____ for 5 years.A.has openedB.has openC.has been openD.has been opened It has been several years since he lived here.怎么翻译? 分析一下它的含义?It is has been four years since he lived here． It has been 4 years since I smoked.为什么翻译成 我戒烟4年了? It has been ten years since I ___(be) a teacher. It has been 5 years since I smoked...我已经戒烟物五年了? It has been many years since I enjoyed myself so much. The man has been dead for ten years(同义句） It”s ten years__the man__