the key to... 意思是“...的答案；...的关键；...的线索”。to 是介词，后接名词、代词、动名词。to 介词短语作 key 的定语，不能使用不定式作定语。

I don't know the key to the puzzle.我不知道这个谜语的谜底。《外研社英语同义词辨析词典》

Confidence is the key to success.信心是成功的关键。《牛津词典》

The driver of the car probably holds the key to solving the crime.这位汽车司机很可能掌握破获这一罪案的关键证据。《牛津高阶英汉双解词典》

Scientists provided the key to understanding the genetic code that determines every bodily feature.科学家们提供了线索来了解决定每个身体特征的遗传密码。《柯林斯英汉双解大词典》

Mutual understanding is the key to bridging the generation gap.相互理解是消除代沟的关键。

网友说的She had the ability to explain things clearly. 这个句子，可以改为 of explaining，但不是 to explaining。