问题补充：
when we use metals it is important to know how they how they react with differet substans,for example,water and oxygen主谓宾补定状表分别是?求援助啊!急~~在线等~~~
答案：
这个句子的大框是：
时间状语从句+形式主语+系动词+表语+作为真正逻辑主语的不定式结构
when we use metals时间状语从句
to know how they react with different substans这不定式结构做整个句子的逻辑主语
is是系动词
important是表语
还要再细分的话就要拆开每个小句来分析了.
先看那个时间状语从句,在这个从句中：
when是时间状语从句的引导词,we是这个从句的主语 use是非谓语动词 metals是宾语
最后那个不定式：know是非谓语动词 how they react是宾语 with different substans是状语,表伴随
======以下答案可供参考======
供参考答案1:
it 是形式主语，how they react with differet substans是真正意义上的主语，is当然就是谓语了，important 是表语。when we use metals是时间状语
供参考答案2:
when we use metals / it is important to know / how they /how they react with differet substans /,for example,water and oxygenit 划分句子
时间状语从句when we use metals ...
主句how they react with differet substans 主语they 谓语react 宾语substan
补语it is important to know
