Question

问：where has he been these days与where has he been in these days中第一个对，告诉我原因

谢谢

Answer 1

不加 in，原因是：规则。
这一类的还有： today, this week, this morning.是人们说话的习惯。按照语法，有些是可以有介词的，也许古老一些的英语曾经有过呢（谁知道？）但是现在：
I will be very busy today 不说 in today.
I won't have any time to go shopping this week 不说 in this week
I'll go to school early this morning 不说 in this morning

Answer 2

矮真的洞特孬