as
as引导原因状语从句,说明比较明显的原因,多用于日常语中,常译为“由于”,从句一般放于主句前.例如：
As he wasn't ready in time,we went without him.
As he is ill,he won’t go.
I must stop writing now,as I have rather a lot of work to do.
As you don't feel well,you had better stay at home.
As it is raining,you'd better take a taxi.
since
since引导原因状语从句,语气比as 强.强调说话者和听者双方都知道的原因,常译为“既然”,一般用在这样的句子里.例如：
Since you are not feeling well,you may stay at home.
Since everybody is here,let's begin the meeting.
Since I am a boy,let me carry the case.
Since/As you are busy,you had better not go with us.
Since we have no money,we can't buy that vase.
[注] now that也有此意.如：
Now that you have finished your work,let’s go now.