1.B(从句中用陈述语气）
2.其实这两个句子是不一样的,我来分析一下.
It is only when you devote yourself to it that you will manage it.
句中when引导一个表语从句,而其后的that引导一个结果状语从句,因为这两个从句之间存在因果关系,类似于汉语中的只有.才能.
It is when I came back that I received your letter
这个句子本身固然是正确的,但是总的来说他是一个强调句,
改为陈述句就是,I received your letter when i came back.对时间状语进行强调,就成了It is when I came back that I received your letter.
两者的区别很简单,从句意上或者从因果关系上都能很好的区别.