I will demonstrate that is how I love you.

问题描述:

I will demonstrate that is how I love you.

你给出的句子有问题,多了一个 is,不过可以忽略不计,我懂你想说什么.
【参考译文】
我将证明我有多么爱你.
或者你的这个句子可以改成What I will demonstrate is how I love you.
我将证明的是我有多么爱你.