新概念英语二册 Lesson89 A Slip of The Tongue 第（5）套免费PPT课件下载

though,although,as ,while;
though和although两个单词本身就表示“虽然….，但是…….,” 所以绝对不能和but 连用。
They are my relatives though I don’t know them.
尽管我不认识他们，他们也是我的亲戚。
as; as 也可以引导让步状语从句，意思 
与though /although  相同，用as  引导让步状语从句，通常放在句首，且主谓要倒装。 As 前面的词可以是形容词，副词，也可以是动词，名词
Rich as he is, he is not happy=although he is rich, he is not happy.
虽然他很富有，但他并不幸福。