I know what to do.
这里的what to do是疑问代词 + 不定式，语法书上说可以将 what to do 看作是名词从句直接做know的宾语，今天我看到了这句话：
I always forget to tell you that how lucky I am to encounter you.
这里的how lucky I am to encounter you.就不可以看作是名词从句了吗？还要用that来引导宾语从句吗？
I always forget to tell you that how lucky I am to encounter you.
我赞同网友douya0808的看法：that是多余的。也就是说，上述句子是个病句。
简单地说：名词性从句，包含宾语从句，其引导词要么用that，要么用wh-word（含whether和how，how也有一个w和h）。也就是说：that和wh-word不可以同时引导一个名词性从句！
（说明：为了叙述便捷，把从属连词、连接代词、连接副词统称为“引导词”。）
比如：
1. 陈述一个事实，用that引导:
My friends tell me that you have been unwell recently.
2. 以下句子中的宾语从句用了wh-word引导，不可再用that：
Tell me how you did it.
Tell me when you were born.
Tell me where you went yesterday.
Tell me what you are doing now.
I want to tell you how I miss you. （本句中的how I miss you也是感叹句，跟提问的句子是一回事。）
3. 但，假如宾语从句中又含有一个主语从句，则可以用that + wh-word引导：
I know that what you are doing is very important. 我了解你们正在做的事情是尤为重要的。
I hear that how he succeeded is still a mystery. 我听说他是怎么样成功的仍然是一个谜。
