定语从句一般不省略.
状语从句的省略分两种情况：
1.主句主语和从句主语一样,从句动词有Be动词.
When he was watching the movie,he heard a loud sound.
----When watching TV,he heard a loud sound.
2.从句主语是it,有Be动词.
If it is necessary,I will buy this product.
---If necessary,I wll buy this product.
As引导的定语从句并不省略,我举个例子：
As you know,he is a pretty loyal soldier.
As 引导定语从句并且在从句中做动词know的宾语.