1.Once year(has passed)since we last met.为什么不用have passed?

1.Once year(has passed)since we last met.为什么不用have passed? Five years has passed since we met last .为什么不用have passed? one year has passed since we last met. It has passed 10 years since we met last time. three years have passed since they met last time. one and a half years (have/has) passed since we last meet. Four months __ passed by since we met last time.Four months the year that has passed since the downfall of apocalypse! Several years has passed since we started our middle school ten years have passed还是has passed 2 years has passed还是have passed three centuries has passed since that accident happened.为什么用