这么回答你的问题：就你的句子而言，If I knew..., 意味着事实上I don't know. 那么knew的宾语从句就是我想知道的事实，所以从句用陈述语气。我造一个句子如下：

先看事实：I don't where he is. That's why I'm asking you now. 这二个句子都表示现在的事实。我们做一个与现在事实相反的假设：

If I knew where he is, I wouldn't be asking you now.

knew 和 wouldn't be asking都是与现在事实相反的虚拟语气动词形式。但where he is. 却是表示现在事实的疑问分句，所以用陈述语气。