有读者问这样一句话：
(1)“Well，” said St. John, laughing, a thing I had never known him do before.
请问动词 know 后的“宾语+不定式”结构中的不定式是否也可带 to?但何时又不带to?
英语里有两三个动词后的用作宾语补足语的不定式往往可带 to， 亦可不带 to。最常见的是动词 help，我们已较详细地讨论过。另一个较常见的动词就是 know。关于 know 后面用作宾语补足语的不定式，似乎不可笼统地说它可随意地带 to 或不带 to。
我们知道，动词 know 可意指“知道，认为”，亦可意指“经历，看到，听到”。在后接用作宾语补足语的不定式时，二者是有显著区别的。总的说来，前者（常用现在或过去一般时）之后的不定式应带 to，如：
(2)I know him to be ill.
(3)They knew the man to have been a spy.
应该指出，上述这种句型较正式、文气，所以有些英语学习词典未收纳。
具有“经历，看到，听到”意义的动词 know（多用完成式，有时亦用过去式）后的不定式则可带 to， 亦可不带 to。据一位语法大师说，不带 to 的情况要多于带 to 的情况，但这恐是指英国英语而言。有一位美国语法学家则说带 to 的情况较为晚近，乃是受到法语和拉丁语的影响所致，所以虽不能说是生硬或文气，但较为高雅，多用于严谨的文体中。现举一些实例如下。
关于不定式不带 to 的情况，上述例（1）即是一例，再如：
(4)I have never known him tell a lie.
(5)She had never known them fail .
(6)I have known strange things happen .
(7)He had known him often refuse to be concerned.
(8)I never knew her complain .
再看一些不定式带 to 的例证：
(9)I have never known her to tell lies.
(10)I have known them to strike their children in front of visitors.
(11)I had never known him to pass the garden-gate before.
(12)I never knew her to come on time.
(13)Did you ever know him to wear a black over his eye?
有些辞书还明确举出一些不定式可用亦可不用 to 的例证，如：
(14)I've never known it (to)snow in July before.
(15)I never knew him (to)do such a thing.
(16)I have known her (to)lie .
但须指出，如果动词 know 用于被动语态，则不论其意为“知道，认为”或是“经历，见到，听到”，其后的不定式皆须带 to，如：
(17)He is known to be a good man.
(18)He is known to have been a spy.
(19)Even small amounts of these substances are known to cause skin problems.
(20)She has never been known to lose her temper.
此外，下面一些情况亦一般皆须用带 to 的不定式：
(21)I have never known her to have failed .（不定式的完成式一般皆带to）
(22)I have never known him to be dishonest.（动词 know 即使用完成时态，其后的不定式 to be 亦不可省去 to）
然而，下面一句中的不定式的否定式则通常不带 to：
(23)Have you ever known him not come ?
请注意下面一些句中的不定式（皆须带 to）有点特殊：
(24)I know him to speak to .我和他仅仅是熟到打打招呼而已。
(25)I know him only to say hullo to .
(26)I know him by sight, but not to talk to .
