1. I understand 接that 从句时，一般表示“听说，获悉”。指从获得的情报、消息、情况中得出结论、推测，一般用于比较正式的场合。除了接that 从句作宾语外，还可接以动词不定式充当补足语的复合宾语。例如：

I understand that you will be moving here soon.

我听说你就要搬到这儿来住了。

I understand that you are now married.

我获悉你现在已结婚了。

既然如此，网友的句子就没有这层意思。而且是肯定意义，不能跟can't连用。

2. 当然，understand 如果接wh-word 引导的名词性从句，则是“理解，明白”之意。例如：

I don't understand what you're talking about.

我不明白你在说什么。

3. understand 接动名词作宾语时，该宾语常可带名词或代词的属格或宾格表示其逻辑主语。例如：

I really can’t understand  you treating her like that.          

我真不理解你会那样对待她。

   我认为 A. you treat 和 D. you treating 都是正确的。例如：

    I just can't understand his taking the money.我就是不明白他为什么会偷钱。/ I can understand you feeling angry or that you feel angry. 你生气我能理解。（摘自牛津 外研社《英汉汉英词典》，外语教学与研究出版社出版发行，2010年11月第1版）

I really can’t understand you treating her like that. 

参考译文：我真的不理解你为什么像那样对待她。

A. you treat B. you to treat C. why treat D. you treating 

以上属于后接动名词复合结构作宾语

understand sb/one's doing sth

I can't understand neglecting children like that.

对孩子这样毫不经意,我不能理解。

I can't understand his resigning his job.
我难以理解他辞职的原因。

I just can't understand his taking the money.
我真不理解他为什么接受那笔钱。

解答如下：

(1)can't understand +wh-从句 后面通常接疑问分句

You don't understand what a difficult position I'm in.

你不了解我的处境是多么困难。

She doesn't understand how to look after you.
她不知道应怎么照顾你。

I can't understand neglecting children like that.

对孩子这样毫不经意,我不能理解。

I can't understand his resigning his job.
我难以理解他辞职的原因。

I just can't understand his taking the money.
我真不理解他为什么接受那笔钱。

(2)Do/Does +主语 understand+wh-从句后接疑问分句

Do you understand how this machine works?
你知道这台机器是怎样运转的吗?

Do you understand what to do next?
你们知道下一步该做什么吗? 

Do you understand how to repair the machine?
你懂得如何修理这台机器吗? 

(3)understand +wh-从句 肯定也接疑问分句 

I understand how you feel.
我理解你的心情。

(4)understand +that从句

Little by little you'll understand that you've taken the wrong road.

你渐渐会明白你走错了道路。

You will of course understand that my point of view is different.
你当然清楚,我的观点是不一样的。

We understand that you'll be returning next year.
我们知道你打算明年回来。