网友的句子有误：of 应改为to。

We don't even know the extent to which that sort of covert influence in other countries domestic politics has gone on.

【翻译】我们甚至不知道这种对其他国家国内政治的隐性影响程度有多大。

which 之前加to，是因为先行词是the extent（to the extent 到达那种程度）的缘故。

把to 提到 which 之前，是因为介词to 跟 the extent 搭配紧密，不可分割。此外，即便能放在句末，但句末有一个 on，考虑结构紧凑，也要提前。

先行词是the extent，后跟 to which 的例句：

No one can forecast the extent to which traffic will grow.

没有人能预测将来交通流量会增长到什么程度。

She didn't realize the extent to which she had been distracted. 

她没有意识到她心烦意乱的程度。

The extent to which the Court will modify these doctrines is, as yet, unclear.

法院对这些原则修正的范围现在尚不清楚。

The extent to which women have lost their rights in Iraq is shocking.

伊拉克妇女丧失权力的程度令人震惊。