since 与now that的区别

1.since（既然）introducing known reason
当原因是显而易见的或已为人们所知,就用 since
例：Since you can't answer the question,I'll ask someone else.
既然你回答不出这个问题,我就问别人了.
2.now that（由于…,既然…）,obvious reason.
突出事件性 ,表突发情况,用now that,that可省略.
Now that you are busy,let me do it for you.
既然你忙,就让我给你做吧.