这个很容易区别.
如果连接引导词前面的主语是地名,用where；是人用who；是事件的话就用that.为示例简明,用最简单的句型.例如：
The house where is near by the lake is my family.(这里的主语是 The house,所以用表示地点的where来引导)
The girl who is dress in red is my sister.(这里的主语是 The girl,所以用表示人的who来引导)
I told you that never touch my cup.（这里要指代的是I told you的事情,所以用that来引导从句）