it is since为什么不用完成时就是it is 一段时间+since+过从,为什么be动词是现在时.不是完成时?

it is ...since ..为什么不用完成时it is 100 years since Qinghua university was found. it is since为什么不用完成时就是it is 一段时间+since+过从,为什么be动词是现在时.不是完成时? it __ about fifteen years since he __ his hometownA.is;leftB.has been;leaves为什么有since的时候不用完成时 It is over a year since I last wrote to him.为什么前面不用完成时?It has been over a year . It is 3 years since he joined the army.为什么用is不用has been it is four years since I joined the army.这里的is,为什么不用was呢? 为什么It is two months since she left Beijing用is不用has been It __four years since i joined the army.这里的is,为什么不用was呢? 分析这个英语句子的时态和句意!it is／was 3 years since he came here.since不一定和完成时连用吗?用is或was有什么区别 为什么since不能用做that? it is/has been+时间段+since since后面接完成时还是一般过去?求例子详解!拜谢~ It is ten years since Miss Green returned to Canada为什么不用It has been ten years since Miss Green returned to Canada It __ten years since he left Shanghai.为什么用is,不用had been.意思是“从那时有多久”？ 现在完成时 自他参军至今已有十年了.It is ten years since he jioned the army.since前为什么不用现在完成时。 2.It is six months_____I last saw him.A.after B.for C.with D.since 为什么用since 而不用after 为什么不用since but ever since he moved in,he has had trouble with cars.前面一句为什么不用完成时. it's five years since I _you last A.met B yes,you're right C .thank you D.she is fine为什么选A?since不是完成时的标志吗?it's five years since I _you last timeA.met B,meet C have meet D had meet it __ ten years since he __hereA.is;left B.has been;had left为什么是A啊,有since不是应用完成时吗?