英语中的因为都是从属连词?

看下面的句子：
He was ill,so he didn't come
可以改写成以下“表示原因”的状语从句（从属句）：
1 As / Since he was ill,he didn't come.
2 He didn't come because he was ill.
可以改写成以下“表示并列”的简单句：
He didn't come,for he was ill.
(for在这里的语法表现如同and,but,or 等等,表示并列,不表示从属).