原因状语从句as和since有略微区别，可互换。例如
As/Since/Seeing that you are gere，you may
as well give me a hand.
既然你在这里，你就帮我个忙吧。
As/Since/Seeing that Tom knows French,he'd better do the talking.
既然汤姆懂法语，最好让他来谈。
as 引导原因状语从句时表示附带说明的“双方已知的原因”， 语气比since弱， 较为正式， 位置较为灵活(常放于主句之前)。
例如： As it is raining, you’d better take a taxi. 既然在下雨， 你最好乘出租汽车。// As you are tired, you had better rest. 既然累了， 你最好休息一下。
I went to bed early, as I was exhausted. 我睡得早， 因为我筋疲力尽了。
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”)， 较为正式， 语气比because弱。
例如： Since you are free today, you had better help me with my mathematics. 既然今天你休息， 你最好帮我补习数学。