Four years()since I ()the army 求分析为什么是 has passed joined而不是 passed have joined

It---------- (be) four years since I moved here. It is four years since I joined the army.啥意思 it is four years since I joined the army.这里的is,为什么不用was呢? 下面两个句子有什么区别?It is four years since I knew Candy.It has been four years since I knew Candy这两个句子有什么区别? I _____ the flat since four years ago A.have live B.have had C.bought D.have bought Four years()since I ()the army 求分析为什么是 has passed joined而不是 passed have joined it is four years since i an english teachera become b became c becomes d have became说下为什么 it's years since i my brother left school four years ago（同义句）my brother_______school since four years age. 句型转换she left her hometown four years agoshe()()()()her hometown since four years ago Mr Yao ______ (go) to Amecia four years ago,so he _____ (be) there for four years.My family _____ (live) in this village since I was born.The concert _____(结束） with an English song. ____ four years since I joined the Army.A. There was B. There is C. It was D. It is为什么选d不能选c啊? --four years since I joined the army.A.There was B.There is C.It was D.It is选个为什么请分析C和D --four years since I joined the army.A.There was B.There is C.It was D.It is 选哪个为什么请分析C和D It three years since I smoked.翻译 i have been skating since nine years four years Four Years