(错) For he is ill, he is absent today.
(对) He is absent today, for he is ill.
2) so, therefore
He hurt his leg, so he couldnt play in the game.
a. 两个并列连词不能连用，但therefore, then, yet.可以和并列连词连用。
You can watch TV, and or you can go to bed.
He hurt his leg, and so / and therefore he couldnt play in the game.
b. although… yet…，但although不与 but连用。
(错) Although he was weak, but he tried his best to do the work..
(对) Although he was weak, yet he tried his best to do the work.