简单点说，这是一个从句内，又包含一个从句。 he admited that what he did was wrong. 第一个从句的引导词是that：he admited that sth. was wrong. 然后，应该很好理解了，what是第二个从句(主语从句)的引导词，what he did was wrong. 两个从句整合就是：he admited that what he did was wrong.

"that" is the Introduction of a subordinate clause expressing a statement or hypothesis equivalent to the Chinese 所. "what" has the same meaning in CHinese as "做的一切" or 什么. 他承認他所做的一切是錯誤的 or 他承認他做了什么都是錯了 etc