as作连词的用法：

1、作，当的时候，引导时间状语从句。

例句：As a young man, he was active in sports。

2、as等于since，作既然。

例句：As he wasn't ready in time, we went without him。

3、作，虽然、尽管，引导让步状语从句。

例句：Tired as they were, they walked on。

as作关系代词的用