since和as是从属连词,说明的是已为人们所知和理由.since的语气比because弱,但比as强.since和as引导的从句一般放在句子的开头.
Since he was busy,he didn't come.
他因为忙,所以没来.
As I have a day off today,I'll write to you.
因为我今天有一天假,所以就给你写信.
此外,since也可以述说推断的理.如：
It must have rained,since the ground is wet.
一定下过雨,因为地是湿的.