英语语法分析

If I am the one experiencing the headache, then I have direct evidence of its factualness.

如果我感到头疼，我就有我真的头疼的直接证据。

But if it is you experiencing the headache, I can establish its factualness only indirectly.

但是如果是你感到头疼，我只能间接地确证它的真实性。

If it actually exists it must be somewhere, and, assuming its place to be accessible to you, you can verify its factualness by direct observation. Take the case of the White House.

如果它实际存在，它必定在某处，而且，假如那个地方你很容易进去，你可以直接观察查证它的真实性。