jack was soon told why what he did was not necessary

弄明白整句话的意思就可以了,翻译如下：
杰克很快就被告知了【为什么他所做的是不必要的】
此句等同于 Jack was soon told 【the reason for】 (what he did was not necessary).Why表示括号内从句所述事件的原因.
来点更复杂的例子e.g.：
1、He wonders why what he did irritated his father.
他想知道为什么他的所作所为激怒了他的父亲.
2、I don't know when what he was doing began to worry his girlfriend.
我不知道他的所作所为是在什么时候开始让他的女朋友感到担心的.