1) for是并列連詞，不能置于含兩個并列分句的句子的句首，只能將其放在兩個分句中間。

判斷改錯：(錯) For he is ill， he is absent today.

(對) He is absent today， for he is ill.

2) so為連詞， therefore一般為副詞。例如：

He hurt his leg， so he couldn’t play in the game. 他的腿受傷了，不能上場。

I think; therefore I exist. 我思故我在。