now that, as, since 在引导原因状语从句的区别是有何区别？请多举几个例子, 谢谢!

(1)now that（既然）的用法： now that同since相似，语气较弱，强调人们已知的事实。 

Now that you are busy, let me do it for you. 

既然你忙，就让我给你做吧。

She is much more mobile now that she has a car.
她现在出去方便多了，她有了一辆汽车。

Now that you are a college student, you should learn to be independent of your parents' help.
既然你已是个大学生了，就应当学着独立，不靠父母的帮助。

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。

Now that I'm 90, I find the exertions of travelling too great.
我已经年届90, 出远门已力不从心。

Now that I've paid the tax demand and the children's school fees, I'll be head-over-heels in debt for the next six months.
由于我已缴了应纳的税和孩子们的学杂费，在以后的6个月中我将债台高筑。

Now that she has a job in France she can gratify her desire to see Europe.
既然现在她在法国工作，那就可以满足她游览欧洲的愿望了。

Now that prices are rising so fast, all my money is draining away.
由于价格上涨太快，我的钱都快花光了。

Now that I am free, I can enjoy music for a while.
既然有空，我可以享受一下音乐。

(2)as意为“因为、由于”，语气比because弱，所引导的从句表示的是不言而喻、显而易见的原因和理由，其从句一般放在主句前。

As  it  was  late  ，  we  came  back  soon.

由于时间很晚了，我们很快就回来。

As rain has fallen,the air is cooler.  

由于下了雨，空气比较凉爽。

As he has not ready in time,we went without him.  

由于他未能及时准备好，我们没有等他就走了。

As this question is of great interest,we will discuss it once again.  

因为这个问题很有意义,所以我们决定再讨论一次。

I can't come tonight,as I'm going to a concert.  

今晚我不能来,因为我要去听音乐会。

I am going to bed,as I am very tired.  

我疲倦得很，要去睡了。

(3)since（既然）语气较弱，强调已知的事实。 

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。

Since it is late,I shall go home now.  

由于时间晚了,我现在要回家了。

Since you are so sure of it, he'll believe you.  

既然你对此这么有把握,他会相信你的。

(4)for（因为）的用法： for是并列连词，表示推断的原因，或对前一分句补充说明理由。 

It must be morning for the birds are singing.

一定是清晨了，因为鸟儿在叫。

Something fell in, for I heard a splash.   
有东西掉进去了,因为我听见扑通一声。

The angles are equal, for ABC is equilateral.   
三角相等,因为三角形ABC是等边三角形。

(5)because通常表示直接的原因。because引导的从句表示直接而明确的原因和理由，表示按因果关系的推断，语气很强，用来回答why的问题, 一般位于主句之后。 

We couldn't go out because it was too cold. 

因为天气太冷，我们不能外出。

I study because I want to learn.   
我学习是因为我想学到知识。

Just because they make more money than I do, they think they are so superior.  
只因为他们挣钱比我多,他们就认为自己高人一等。

I bought the house simply because it was large.

我购买了这房子，只是因为它面积大。

An argument was inevitable because they disliked each other so much.

争论是不可避免的，因为他们彼此非常厌恶。

I knew she was joking because she had a big grin on her face.

我知道她是在开玩笑，因为她脸上堆满笑容。