after that和after which这两个表达方式范畴不同，含义也不同。

例1：After that, they settled down. 在那之后，他们定居下来。

that在句中就是个普通的代词，与how dare you say that中的that是一样的。after that不是从句。

例2：Mornings began with three hours of classes, after which children were often treated to playtime in the yard. 早晨先上三节课，在那之后孩子们可以到院子里玩。

which代指“早晨先上三节课”，是个引导词，引导的从句是定语从句。

在定语从句范畴里，虽然that也可以做引导词，但介词后通常不能使用that，所以本句只能用after which不能用after that。