动词know后的不定式符号to的省略问题
当动词know的后面跟有不定式作宾语补足语时（即用于“know+宾语+不定式”），该不定式有时带to，有时不带to，具体用法如下：
(1) 若know为一般现在时，则其后用作宾语补足语的不定式不能省略to（此时的不定式通常为to be型）。如：
I know him to be ill. 我知道他病了。
We know her to be honest. 我们知道她是诚实的。
(2) 若know为一般过去时或现在完成时，则其后用作宾语补足语的不定式可以省略to，也可以保留to（此时的不定式可以是to be型，也可以是to do型）。如：
I never knew him (to) do such a thing. 我从未听说他会干那种事。
I’ve never known her (to) be late before. 我以前从不知道她迟到过。
I’ve never known it (to) snow in July before. 我从未听说过有七月下雪的事。
I have never known a horse (to) eat fish. 我没听过马会吃鱼。
但是，当用作宾语补足语的不定式为完成式时，其中的to不能省略。如：
They knew the man to have been a thief. 他们发现此人曾是个小偷。
I have never known her to have failed. 我从来没发现她失败过。
(3) 当动词know本身用于被动语态时，其后的不定式不能省略to。如：
He has never been known to tell a lie. 从未有人听说过他说谎。
Smoking is known to increase a person’s risk of developing lung cancer. 据说吸烟会增加一个人患肺癌的风险。
People have been known to drive 500 miles just to visit the shop. 听说有人驾车500英里只是为了去这家商店。