前面有人提问2022年高考考试英语全国甲卷（全国卷Ⅱ）阅读理解A中的问题。我在查阅有关原文时碰到了阅读理解B中的一个问题：
Though not known to usetools in the wild, the birds have proved skilful at tool use while kept in thecage.
这个句子中的 Though not known to usetools in the wild 应该是 Though theyare not known to use tools in the wild 的省略形式吧。
我的问题是：
我在前面一个问题（https://www.cpsenglish.com/question/59841）获悉：
know sb do sth 中的动词 know 一般用完成式，但这里仿佛并非完成式啊？
主动式 know sb do sth 中的 to 可以省略，假如变成被动式，其中的 to 就绝对不能省略了吧！
前面几位老师的解答都有道理。
但我想网友的问题也可以从另外一个角度来剖析。
我相信网友肯定熟知下面的句子：
People say he is therichest man in the village.
It's said that he isthe richest man in the village.
He's said to be therichest man in the village.
其实，动词 know 也具备与say一样的使用方法。譬如可以说：
People know he is therichest man in the village.
It's known that he isthe richest man in the village.
He's known to be therichest man in the village.
了解了以上使用方法，网友应该就不会纠结了。
下面再补充几例 sb/sth is known to be / do 的使用方法：
Drinking is known tobe harmful. 饮酒有害。
She is known to beinterested in pop music. 她喜欢时尚音乐是人所共知的。
This drug is known tohave adverse side effects. 大家都知道，这种药具备不好的不良反应。
假如不定式表示的是已经发生的动作或状况，则不定式用完成式：
He is known to havebeen a spy. 大伙都了解他当过特务。
She is known to havebeen married before. 据了解，她以前结过婚。
He is known to havecommitted crimes. 大家都知道他犯下了种种罪行。
