since引導的`原因狀語從句：
since引導的原因狀語從句一般放於主句之前表示已知的、 顯然的理由(通常被翻譯成“既然”)， 較為正式， 語氣比because弱。
例如：
Since you are free today, you had better help me with my mathematics.
既然今天你休息， 你最好幫我補習數學。
Since you don't trust him,you should not employ him.
你既然不信任他，你就不該僱用他。
注意： seeing (that)， now (that)， considering (that)， in that這幾個詞彙與since引導的原因狀語從句意思相近， 都表示“既然”。
例如：
Seeing (that) he refused to help us, there’s no reason that we should now help him.
他既然曾經拒絕幫助我們， 我們沒有理由要來幫助他。
Now (that) you are grown up, you should not rely on your parents.
既然你長大了， 就不應該依靠你的父母了。
Considering (that) everybody is here, let’s begin our discussion.
既然大家都到了，我們就開始討論吧。
In that he is ill, he feels unable to do it.
因為他生病了，他覺得做不了那件事。