Question

we’d better wait till Dec.12th.Tom (have)__his exam by then so he'll be able to enjoy himself.

答案为什么是will have had 请详细说明 谢谢

Answer 1

first, Dec.12th 是将来的时间，所以用will,
second, 在Dec.12th 的这个时间点来开，Tom 的 exam 已经完成了，所有用完成时have had。
这是一个将来完成式的时态。