I don't understand you.

问题描述:

I don't understand you.

I don't your understand
哪个对?为什么?

第一句I don't understand you.是对的,意为:我不明白(听不懂)你在说什么.
第二句助动词否定式don't 后面要跟动词原形作谓语,此句缺少作谓语的动词,本身不成句子.语法错误.