since引导的原因状语从句：
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。
Since you don't trust him,you should not employ him.
你既然不信任他，你就不该雇用他。
注意： seeing (that)， now (that)， considering (that)， in that这几个词汇与since引导的原因状语从句意思相近， 都表示“既然”。
例如：
Seeing (that) he refused to help us, there’s no reason that we should now help him.
他既然曾经拒绝帮助我们， 我们没有理由要来帮助他。
Now (that) you are grown up, you should not rely on your parents.
既然你长大了， 就不应该依靠你的父母了。
Considering (that) everybody is here, let’s begin our discussion.
既然大家都到了，我们就开始讨论吧。
In that he is ill, he feels unable to do it.
因为他生病了，他觉得做不了那件事。