showed:v. 显示；表现；展示（show的过去式）

He showed courage in adversity.

shown:v. 解释（show的过去分词）；给予；表露

What have their researches shown

What have their researches shown

他们的研究证明了什么？

You have shown great strength of character.

showed是show的过去式用做过去时态语句里

shown是show的过去分词用做完成时态语句里