really can't understand you treating her like that英语高手进为什么不是I really can't understand you to treat her like that

really can't understand you treating her like that英语高手进
为什么不是I really can't understand you to treat her like that

没有understand sb.to do sth .
只有：understand sth/sb .
所以：I really can't understand you （treating her like that） 这个分词短语,可以理解成补语.
也可以理解成：
I really can't understand (your/you ) [treating her like that]
中括号内是宾语,而小括号内,是宾语的逻辑主语.
这就像do you mind my/me opening the window 这个常见例句一样了.

treating 在这里表主动，you 和treat 的关系就是主动，而to treat 不定式有表示将来的意味，但根据这句话的意思，这件事已经发生过，所以不用to treat.

句意为：我真的不明白你为什么那样的对待她。注意understand为及物动词，后加名词，没有"～to do sth."的形式。treat要变为动名词形式，又因为“对待”这一动作的发出者与句子主语不一致，所以要加上它自己的逻辑主语“you”