过去将来时中，in和after怎么用？

Question

He said he would go to Beijing in three days.
He said he would go to Beijing after three days.

哪个对，为什么？求解！

Answer 1

我记得在精锐学习的时候，老师说in表示现在的以后将来追问

过去将来时 怎么用？不是一般现在时