状语从句：1.原因状语从句，由because，since，as引导，语气由强及弱，前者可用于任何位置，后两者只能用于句首。(eg:He is absent because he has a bad cold.)       2.结果状语从句，由so，so…that，such…that引导。（eg：It is sunch a small matter that I do not care.)