since 和 as 引导原因状语从句时,有什么区别?

as,because,since都可以表示因果关系,连接原因状语从句,含义是"因为,由于",但它们有区别：because表示的语气最强；as一般放在句首,语气较弱,较口语化；since常常用在书面语中,表示多为对方已知的、或稍加分析便可得知的原因,有时可译作"既然".例如：
(1) He will succeed because he is in earnest.他一定会成功,因为他很认真.
(2) Since you are so sure of it he”ll believe you.既然你对此如此有把握,他会相信你的.
(3) As rain has fallen, the air is cooler.因为下过雨,空气比较清爽.