介词+which和where在那种情况下不能互换

The ladder on which I was standing began to slip 这里的on which 为何不可以用where替换

The ladder on which I was standing began to slip 我站在上面的那架梯子开始滑倒/滑动。

解答如下：

The ladder where I was standing began to slip

The ladder on which I was standing began to slip

备注：第2句要比第1句表达更准确。更具体些。