It is not when to do it but how to do it that is of great importance.
问题描述：
It is not when to do it but how to do it that is of great importance.
It is not when to do it but how to do it （that） is of great important
其中括号里为什么是用that而不是用which?
答
支持楼上的,是强调句型
It is（was）＋被强调的部分＋that（who）＋原句其它部分”来强调说话人的意愿
被强调的部分为“人”时用who,whom,为“物”时用that.
不是什么时候做,而是如何做这一点是非常重要的
强调句型有个特点就是：把"It be…that…"从句中划去,所剩的还是一个完整的句子.
not when to do it but how to do it is of great importance.