It's not necessary to explain the truth that wherever there

It形式主语
to explain the truth that wherever there is spring wind,there is green grass是真正的主语,其中
that wherever there is spring wind,there is green grass是that引导的同位语从句,修饰truth;
又wherever there is spring wind是一个地点性状语从句,修饰there is green grass.

It is + adj. +to do 做什么事是怎么样的
先行词是名词，所以这句话是个定语从句。从句修饰truth。

哪里有春风，哪里就有绿草，这个真理无须解释。

It 为形式主语 真正的主语是 to explain the truth
that引导的从句是truth 的同位语
wherever there is spring wind, there is green grass是状语从句：前面为从句，后面为主句