为什么不是i really can not understand you treat her like that?

这个句子意思不完整,翻译过来就是“我不明白你那样对她?”而应该改为I really cannot understand why you treat her like that?
再问： 可否说成i really understand you treat her like that。对吗？是不是can not understand或者do（es） not understand 或did not understand的后面不能带that引导的宾语从句？