It is the worst weather since 50 years ago为什么不是It has been.不

It is the worst weather since 50 years ago为什么不是It has been.不 It is 3 years since he joined the army.为什么用is不用has been It has been there since a t_ years ago . The picture has been here.It was here two years ago.用since该写 It has been years since I returned home,为什么since用,而不用when?分不 it is 3 years since Tom has been abroad it is two years since I came here还是it has been two years sin her uncle died 10 years ago . it is 10 years since he has be 看到一句It is 3 years since he joined the army.和一句It has been th 请问It is six years since I last met you 与 It has been six yea It has been 3 years since he smoked.为什么这句话的意思是他不吸烟已经三年了? 现在完成时讲解1.She has worked since ten years ago.2.It is ten year