1.because conj. 因为
语气最强，表示因果关系，通常用来回答由 why 引导的疑问句。由 because 引导的分句置于句首时，须用逗号与主句隔开，置于句末时则不用隔开。
Because the traffic is so busy, she failed to arrive at the station on time.
因为路上交通太拥挤，她没能准时赶到车站。
Just because I don't complain, people think I'm satisfied.
就因为我不发牢骚，大家便以为我满意了。
He came back late because there was something wrong with his car.
他回来晚了，因为汽车出了故障。
The team has made some positional changes because two players are injured.
因有两名队员受伤，这个队将队员做了些位置上的调整。
The party went well because everyone entered into the spirit of things.
晚会很成功，因为每个人都很投入。
We couldn't understand them because they were talking in Chinese.
我们听不懂他们在讲些什么，因为他们说的是汉语。
Nothing will be done because no one in authority takes the matter seriously.
什么也办不了，因为掌权的谁也不认真对待这个问题。
I was forced to take a taxi because the last bus had left.
最后一班公共汽车已经开走，所以我只好坐了一辆出租汽车。
She was treated like a freak because she didn't want children.
她因为不要孩子而被当作怪人。
She did it first because she wanted to, and second because I asked her to.
她做那件事，首先是因为她自己想做，其次是因为我要她做。
2.as conj. 因为，由于
如果所述理由是明显的或者已知的，则用 as 较 because 更为合适。
We asked Philip to come with us, as he knew the road.
我叫上飞利浦和我们一道去，因为他知道去的路。
As it's raining, you'd better stay at home.
既然在下雨，你最好呆在家里。
As you're tired, you should rest.
你累了，应该休息。
He puts the ice cream into the fridge immediately, as it melts easily.
他马上把冰激凌放进冰箱，因为它很容易化掉。
They are regularly sent booklets about personal safety, but they barely read them as they have so much paperwork to deal with...
他们经常会收到关于人身安全问题的宣传册，但因为大量文案工作缠身，他们几乎无暇阅读。
如果原因已被人们所知，或不如句子的其它部分重要，就用as或since。Since比as稍微正式一点。As和since 引导的从句一般放在句子的开头。例如：
As he wasn’t ready, we left without him.
由于他没有准备，我们没有带他。
Since I have no money, I can’t buy any food.
既然我没有钱，我不能买任何食物。
3.for conj. [正式] 因为
for 属于并列连词，由它引导的分句表示补充说明性的额外增补的的理由，通常只能置于句末，且必须用逗号与前面的分句隔开。
He must have passed this way, for here are his footprints.
他肯定是从这条路走过去的，这里有他的脚印。
He must be ill, for he hasn't come to work today.
他今天一定病了，因为他没来上班。
I decided to stop and have lunch----for I was feeling quite hungry.
我决定停下来，吃午饭。因我感觉很饿。
He has to work hard, for he wants to support his family.
因为要养家糊口，他必须努力工作。
I believed her ─ for surely she would not lie to me.
我相信她的话——因为她肯定不会向我撒谎。
He had a great desire to have a home of his own for he had always lived with my grandmother.
他强烈渴望拥有一个属于自己的家，因为他一直和我祖母住在一起。
We listened eagerly, for he brought news of our families.
我们急不可待地听着，因为他带来了我们家人的消息。
在（文学）故事中，for有时用于解释或论证：
He seemed to be in need of company, for he suddenly went back into the house.
他看起来需要陪伴，因为他突然回到了屋里。
4.since conj. 因为，既然
当所述原因是已知的客观事实或被当作推断的理由时用 since，语气略强于 as。
Since he is absent, let's begin.
既然他不在，我们就开始吧。
Since it's raining, we'll stay indoors.
既然下雨了，我们就待在室内吧。
Susan must have come back home, since her coat is not in the wardrobe.
苏珊肯定回过家，因为她挂在衣橱里的外套不见了。
Since the juice is quite strong, you should always dilute it.
这种果汁太浓，所以必须加以稀释。
The exhibition is timely, since 'self-taught' art is catching on in a big way.
展会召开得非常及时，因为“自修”艺术正在广泛流行。
Since Mr Bennett was going to need some time off work, he asked for a sick note
因为贝内特先生要请假一段时间，他请医生开了病假条。
Since he was color blind, he favored large, showy flowers
因为他是色盲，所以他喜爱大朵艳丽的花。
Since she knows little Italian, she keeps to herself.
因为不怎么懂意大利语，她很少和人交往。
I'm forever on a diet, since I put on weight easily.
我永远都在减肥，因为我很容易长胖。
He had reason to be pleased with himself, since he was one of only seven out of forty candidates who were successful.
他有理由沾沾自喜，因为他是40个候选人里仅有的7名成功者之一。
We thought that, since we were in the area, we'd stop by and see them.
我们想，既然到了这个地方，就该顺便去看看他们。
He argued that the issue had become moot since the board had changed its policy.
他争辩说这项议题已变得毫无实际意义，因为董事会已经改变了政策。
Since his wife was much younger than he, he was constantly afraid of being cuckolded.
因为他妻子年龄比他小得多，他老是怕戴绿帽子。
汉英大词典
在语气上，because 最强烈，其次是 since 和 as，for 最弱。