----before we depart tomorrow,we should have a wonderfui dinnerparty.Ahad they arrivedBwould they arriveCwere they arriving Dwere they to arrive

虚拟语气,表将来
D
时间
从句谓语形式
主句谓语形式

将来
动词过去式(be用were)
should + 动词原形
were to + 动词原形

would / should / might / could + 动词原形
现在
动词过去式(be 用 were)
would / should / might / could + 动词原形
过去
had +动词过去分词
would / should / might / could have + 动词过去分词

D were they to arrive
were they to arrive=If they were to arrive 是对将来情况的虚拟.
句意:如果他们明天在我们离开之前到达,我们就举行一个精彩的宴会.