同学你好，该知识点来自沪江网校《600分钟搞定高中三年英语【高考一轮复习】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好哦~
since引导的原因状语从句一般放于主句之前表示已知的、 显然的理由(通常被翻译成“既然”= now that )， 较为正式， 语气比because弱。
Since you are free today, you had better help me with my mathematics.
Since you don't trust him, you should not employ him.
Now (that) you are grown up, you should not rely on your parents.

as 引导原因状语从句时表示附带说明的“双方已知的原因”，含有对比说明的意味，语气比since弱， 较为正式， 位置较为灵活(常放于主句之前)。
As it is raining, you’d better take a taxi.
As you are tired, you had better rest.
I went to bed early, as I was exhausted.
以上，祝同学学习愉快~