Where did you go yesterday?
不加to
有怀疑?下面来举个例子：
I went to hospital yesterday.改为特殊疑问句
Where did you go yesterday?不加to
你问我为什么?我以我的理解来跟你解释下吧,语法上 where 是疑问副词
Where did you go yesterday?换成陈述句来看你就明白了
you went where yesterday.
因为where是疑问副词（上面说了）所以没问题.
如果要加to 怎么办 to 后面是加名词的
所以应该是 you went to someplace yesterday.
不知道我这样说,你明白了没有.