为什么不用since 

为什么不用since  it is ...since ..为什么不用完成时 (英语)这里为什么用since不用till? it is since为什么不用完成时 为什么这里要用since和as,而不用because? 为什么不用过去时  It has been years since I returned home,为什么since用,而不用when?分不 ___ you are very tired, take a rest.为什么用Since,而不用if 1.Once year(has passed)since we last met.为什么不用have passed? ___ you are very tired,you can take a rest.为什么用Since,而不用for Three years_____（pass） since he came to china.中为什么不用被动语态 It __four years since i joined the army.这里的is,为什么不用was呢?