请翻译一下：我知道他完成了工作。

I know that he has finished his work.
从句里面的动词和主句里面的动词是不会冲突的.
本句是一个宾语从句，其中that he has finished his work是从句。整个从句做主句的宾语。that在本句中是关系代词。

时态应该用过去时吧
i knowed that he had finished his work.

I know that he has finished his work.
他完成工作已经是过去的事,故用现在完成时.

I knowed that he had finished his work.

I knowed he's finish his work.