【问】 Tired____he was，we decided not to disturb him.

A.though B.although C.as D.like(1984年攻读硕士学位研究生入学考试英语试题)。

我不明白这里为什么必须选C，而不能选A。tried though he was不是一种很常见的形式吗?

【答】 从这个句子的内容看，本题的从句只能是原因状语从句，而不可能是让步状语从句。tried as he was可表原因，亦可表让步。tried though he was则只能表让步。

只有在让步状语从句里，这种特殊语序中的as才是可以用though来替换的：

1. Tired as they were(=Tired though they were) ，Jurgis and Ona sat up late. (Sinclair)

尽管朱尔金斯和奥纳很疲惫，但是他们很晚才睡。

2. Poor as he was (=Poor though he was) ，he was honest. (Eckersley)

虽然他很穷，但是很诚实。

3. Much as we may pride ourselves on our good taste，we are no longer free to choose thethings we want，for advertising exerts a subtle influence on us. (New Concept English.Book 3. L. 26)

虽然我们对自己有很好的鉴赏力而感到自豪，但是我们再也不能自主地来选择自已想买的东西了，因为广告在向我们施加微妙的影响。

(参见much as，though²…as he is)