去掉to 原因是: Where are you going this afternoon?句子变成陈述形式就是 you are going (to) somewhere 你今天下午去哪儿?由于where 是副词 所以不能用to,然后把where 提前,就成了Where are you going this afternoon?
go 不及物动词,后面接名词的时候加to.接副词的时候不加to. 如;go to school,go home. where 是副词,就不用加to 了。 应该为 where are you going this afternoon?或者 Which place you going to this afternoon?