为什么不用since

为什么不用since it is ...since ..为什么不用完成时it is 100 years since Qinghua university was found. since when do you guys know each other为什么不用过去式这什么这个地方不用过去式, since john is in rather poor health,he can hardly resist catching colds.since 为什么不用although He lived in Beijing since he was born为什么用过去式was,不用过去分词been he lived in china when he was born 为什么不用since Long time no see!Oh it(seems)like years since I last saw 为什么不用looks Long time no see!Oh it(seems)like years since I last saw 为什么不用feels Five years has passed since we met last .为什么不用have passed? ever since she came to china,为什么came不用第三人称单数形式呢? Three years_____（pass） since he came to china.中为什么不用被动语态 It is 3 years since he joined the army.为什么用is不用has been He has lived here since 1992.这里为什么用has lived而不用has been living? It __four years since i joined the army.这里的is,为什么不用was呢? 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 two months has passed since he joined the army为什么用has而不用have 英语翻译为什么用until 而不用when,或since,或after