mary didn't go to school,__she was ill. 用for 还是because

Mary didn't go to school, because she was ill.
or you can say:
For being ill, Mary didn't go to school.
Therefore, in this case, the proper conjunction is because, not for.

for，两个都是引导原因状语从句，但前者和主句用逗号分开，后者则不需要。

我回答晚了.只能给你点例句增加点竞争力了!
---------------------FOR-------------------------
He is not the God of the dead,but of the living,for （to him） all are alive.
神原不是死人的神,乃是活人的神.因为在他那里,人都是活的.（那里或作看来）
--------------------BECAUSE-------------------
Put more water in the pot because it boils down
壶中多放些水,因为总是要煮掉一些水份.
从你的例子来说选FOR关键是看那个逗号.
另外because可以放句首!For作为因为意思的时候不能放句首

_,for_;_because_