(1)know一般现在时，知道，确信，其后面to be, to不能省略。

We know him to be brave. 我们都知道他很勇敢。

I know him to be an honest man. 我确信他是个诚实的人。

(2)know过去时或完成时态，见到过，听到过。其后面to be以外的不定式，to带不带都行。

I never knew him (to) act without thinking.我从未发现他不经过思考便行动。

I’ve known him (to) run faster than that. 我听说他跑得比那还快。

I have never known a man (to) die of love, but I have known a disappointed lover (to) lose weight.我从未听说过恋爱死人，而只看到过失恋的人消瘦。

Have you ever known her (to) lose her temper?你有没有听说过她发过脾气。

l  在这种时态的句子中，若不定式是be动词或者完成式时，不定式to不能省略。

I have never known him to be too busy to study. 我从未看到过他太忙没时间学习。

I have never known her to have failed. 我从未见她失败过。

(3)know被动时，不定式必须带to。

He is known to be honest. 人们都知道他是诚实的。

She is known to be good at telling lies. 人们都知道她擅长说谎。

He was known to have taken part in the Long March. 人们都听说过他参加过长征。