In the early morning, the food that had been laid out for the dead is thrown into a river or into the sea as it is considered unlucky for anyone living to eat it.
as it is considered unlucky for anyone living to eat it.这个状语从句中：
我的理解是：it 主语，指的是food；is considered是谓语被动；unlucky是主语补语；for anyone living to eat it是状语；
我的疑问是：在for anyone living to eat it这个部分，我理解为状语
书上的讲解：但我查了2本书，是如此剖析的：it是形式主语，for anyone living to eat it是实质主语。
这种剖析，到底那种是正确的呢？for anyone living to eat it是实质主语，有点不可以理解？
书上的分析是对的。it 是形式主语，for anyone living to eat it 是实质主语，这是一个带逻辑主语（anyone living）的不定式短语，等于：
For anyone living to eat it is considered unlucky.
活着的人吃了这类食物是不吉的。
下面助你引用 Swan 在 PEU 中的两段有关讲解，供参考：
