it is only because they konw what they are for that thay are

it is only because they konw what they are for that thay are working with such admirable zeal.
句子意思没有问题,语法上分析不出来
it是形式主语,实际的主语是哪部分only because they konw what they are for
that 是表语从句的开头,用主系表表达了because they konw what they are for ,thay are working with such admirable zeal.
以上分析有问题吗,为什么要用主系表+形式主语的形式