(1) ①now that表示原因，意思是“既然，由于”，口语中that可省略。(另见in view of条)

• Now (that) you are well again, you can go on with your work.
  既然你已经痊愈了，就可以继续你的工作了。
• Now that you have the chance， you'd better avail yourself of it.
  既然有了这个机会，你最好利用好它。

②now that也可以表示时间，意思是“现在，当……，而这时”。

• Now that you are a big boy, you must behave better.
  你是一个大孩子了，你必须表现得更乖些。

 (2) seeing/considering that 表示原因，意思是“鉴于……”，多用于书面语。

• Seeing/Considering that everyone is here, we may as well begin the meeting.
  鉴于大家都到了，我们就开始开会吧。
• Considering that this is your first attempt, it is very satisfactory.
  鉴于这是你的第一次尝试，还是非常令人满意的。

 (3) since 也可表示原因(另见in view of条)，意为“既然，由于，因为”，与上述词语无甚差别。

• Philip was not prepared for the question, since in fact he had not made up his mind.
  由于菲利普还没有拿定主意，他就没有准备好应对这个问题。
• Since you are going, I will go, too.
  既然你要去，那我也去。