原因状语从句as和since可以互换吗

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。

一般可以互换的！

since 引导的原因状语从句表示人们的事实，是不需要强调的原因，翻译“既然” ，通常放在句首。如： Since we don't have class tomorrow , why not go out for a picnic?

as 的用法和since 差不多