he will have arrived by this time tomorrow.为什么要用现在完成时?

我举个例子：
I will go to school when he has arrived by this time tomorrow.
当他明天这个时候到的时候,我就会去上学去了.
如果你不变成现在完成时,一看就不对：
I will go to school when he will have arrived by this time tomorrow.
It will rain tomorrow.
I will take a umbrella with me if it rains tomorrow.
第一句是将来时,到第二句就成了一般现在时,因为它成了条件状语从句.

这是现在完成时吗？这是将来完成时，或者更准确的说法：完成时的将来表达法。它表示在将来某一时间结束之前，另一个动作结束所造成的结果或者影响。例如，
By the end of this year, we'll have learnt 3000 English words.

在少数情况下，如果强调动作完成的状态，也可以用现在完成时。例如： She will be forty by the time she has finished her PhD. We won't know

首先大时间是将来时
然后 by the time(by this time)一般与完成时 连用
所以用 将来完成时