The most sadness thing is i can't face to myself.这句话语法对吗？翻译成：最悲哀的是我不能面对自己。

sadness是名词，不是形容词，所以不能说sadness thing，痛苦的形容词是painful或者sad。另外face要直接加myself，去掉中间的to。还有从句中的that是不能省略的。所以这句话应该是：
The most sad thing is that I can't face myself.
最悲哀的是我不能面对自己。

The most sadness thing is that i can't face with myself
因为thing在从句中做主语 所以that不能被省略
而且面对...不是face to 是face with

sadness是名词形式，用most来修饰行不通的……形容词最高形式应该是sadest。“is”后边从句最好加上that。“面对自己”直接说face myself就可以了，不用加介词。
因此全句改成The sadest thing is that I can't face myself.
差不多就通顺了……

从句后面要加that