it is the third time _______late this week.为什么不用that you are而用when you have been

to be
如果不考虑人称,只有填to be 才是对的,意为
这周第三次迟到了.

因为是从星期一开始到现在已经迟到了3次了,现在完成时的含义:从过去某个时间(这里是星期一)到现在为止已经完成的动作.相似的用法:He has written three emails to him this week./This is the second time I have been here.

因为 it is the first (second, third, fourth) time that...是英语中一个常用句式，其中的 that 从句习惯上要现在完成时。又如：
It's the first time I've come here. 这是我第一次来这儿。
It’s the first time she has driven a car. 这是她第一次开车。
It’s the second time this has happened. 是第二次发生这样的事了。
That’s the third time he’s phoned her this evening. 这是今晚他第三次给她打电话了。
若主句谓语改为过去式(was)，那么that从句则用过去完成时。如：
This was the first time he had ever been late. 这是他第一次迟到。
It was the second time he had been out with her. 这是他第二次和她一道出去。
That was the third time he had entered the house. 那是他第三次进入这座房子。

你要问啥呀?不懂....

reach late 迟到