20.5 表原因关系

1) for
判断改错：
　 (错) For he is ill, he is absent today.
　 (对) He is absent today, for he is ill.
　 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。

2) so, therefore
　　He hurt his leg, so he couldn't play in the game.

注意：
　 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 couldn't 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.