1) for是并列连词，不能置于含两个并列分句的句子的句首，只能将其放在两个分句中间。
判断改错：(错) For he is ill， he is absent today.
(对) He is absent today， for he is ill.
2) so为连词， therefore一般为副词。例如：
He hurt his leg， so he couldn’t play in the game. 他的腿受伤了，不能上场。
I think; therefore I exist. 我思故我在。