were to do/should do用于对将来时态的虚拟，都可以用，都是虚拟，意思上略有差别，should多用于“万一、竟然”这个意思If he were to come, what would you do?如果他来，你怎么办？If it should rain, take an ambrella.万一下雨，带把伞
