应该是I don't understand what you mean。这是宾语从句。
用作宾语的从句叫做宾语从句(object clause)。引导宾语从句的关联词有从属连接词、疑问代词、疑问副词、缩合连接代词、缩合连接副词等。如：
He told us he felt ill. 他对我们说他感到不舒服。(关联词是从属连词 that)
I know he has returned. 我知道他已经回来了。(在非正式文体中关联词that被省去)
That he ever said such a thing I simply don’t believe. 我简直不相信他曾说过这样的话。(that从句位于句首时，that不可省去)
We decided, in view of his special circumstances, that we would admit him for a probationary period. 鉴于他的特殊情况，我们决定应允他一段试用期。(主句谓语动词decided与that从句之间有插入语，that不可省去)
I doubt whether he will succeed. 我怀疑他是否会成功。(关联词是从属连接词whether，当doubt用于肯定句中时，后面的宾语从句常用从属连接词whether或if引导)
I don’t know if you can help me. 我不知道你能否帮助我。
Who or what he was, Martin never learned. 他是什么人?他是干什么的?马丁根本不知道。(关联词是疑问代词who和what。从句位于句首是为了强调)
I wonder what he’s writing to me about. 我不知道他要给我写信说什么事。(关联词是疑问代词what)
I’ll tell you why I asked you to come. 我会告诉你我为什么要你来。(关联词是疑问副词why)
You may do what you will. 你可做任何你想做的事。(关联词是缩合连接代词what)
I should like to see where you live, Jon. 我想去看看你住的地方，乔恩。(关联词是缩合连接副词where)
宾语从句亦可用作介系词的宾语，如：
He was deeply displeased by what had occurred that day. 他对那天发生的事感到很不快。(what引导的从句是介词by的宾语)
I walked over to where she sat. 我走向她坐的地方。(where引导的从句是介词to的宾语)
I am curious as to what he will say. 我很想知道他要说什么。(what引导的从句是复合介词as to的宾语)
Your success will largely depend upon what you do and how you do it. 你是否成功将主要取决于你做什么和怎样做。(what和how引导的从句是介词upon的宾语)
有时介系词可以省去。如：
I don’t care (for) who marries him. 我不管谁跟他结婚。
Be careful (as to) how you do that. 你要注意做这件事的方式。
有时全句可用引词it作为形式宾语。如：
He made it quite clear that he preferred to study English. 他很明确地说他宁愿学习英语。(真实宾语that从句前有形式宾语it)
You may rely on it that I shall help you. 你可以指望我会帮助你。(真实宾语that从句前有形式宾语it)
有时现在分词亦可后跟宾语从句，如：
He has just gone away saying that he will return in an hour. 他刚走，说他一小时后回来。(that引导的从句是现在分词saying的宾语)
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