Question

______ before we depart the day after tomorrow, we should have a wonderful dinner party.

A. Had they arrived B. Would they arrive
C. Were they arriving D. Were they to arrive
为什么不选C，而是选D

Answer 1

你好,这是与将来事实相反的虚拟语气,并且省略了IF,
与将来事实相反的虚拟语气的构成如下:
IF 从句用:1. should +do
2. were to do
3.动词的过去式
主句用could /should/might/would+do
当IF从句的谓语是should/were 时，可以if 把省略，把should/were 提前，构成省略倒装
If I were you, i would help her.
Were I you, i would help her.

Answer 2

正确答案 ：D 虚拟语气中虚拟将来的句型，外加倒装句结构。
原本应该是这样的句型 If they were to arrive before we depart the day after tomorrow, we should have a wonderful dinner party. 把第一个if去掉，were提前到they之前做倒装。

Answer 3

这是对将来表示的虚拟语气，固定表达是were to do