although, though引导让步状语从句，主句不能用 “but”.
如：Although they lack official support, they continue their struggle.他们虽然没有得到官方的支持，但仍然继续奋斗。
as虽然，尽管。引导让步状语，常放在作表语、状语的形容词、名词、副词后面。
如：Strong as you may be, you can not lift it. 尽管你可能很有力气，你却无法把他提起来。
Hard as he tried, he was unable to make much progress.他虽然竭尽全力，但是没有取得多大的进步。
