because
表因果关系的语气最强，用来回答why的问句，所引出的原因往往是听话人所不知道或最感兴趣的。
you want to know why i am leaving? i am leaving because i’m fed up with the job and boss.
你想知道为什么我要离开?我走了,因为我厌倦了工作和老板。
my friends dislike me because i’m handsome and successful.
我朋友不喜欢我,因为我长相英俊,事业有成。
在主句是否定的情况下，because从句的理解有两种情形。
i didn’t attend the meeting because he was present too.
可能有以下歧义：
因为当时也有他出席，所以我没有去参加这个会议。
我出席这次会议并不是因为有他参加。
i didn’t marry her because i lived her.
因为我爱她，所以我决定不跟她结婚。
我并不是因为爱她才和她结婚。
since
表示人们已知的事实，不须强调的原因，故常译成“既然……”，通常放在句首。
since monday is bob’s birthday, let’s throw him a party.
既然周一是鲍尔的生日，那我们就开以舞会。
since everyone is here, let’s get started.
既然所有人都在，那我们开始吧。
since you are free tonight, why not drop in and play chess with me?
既然你今晚有空,为什么不下来,和我一起玩象棋
考题
you are leaving tomorrow, we can eat dinner together tonight. (99-06 cet-6)
a.for
b.since
c.before
d.while
as
as与 since用法差不多，所引出的理由在说话人看来已经很明显，或已为听话人所熟悉而不须用because加以强调。
she didn’t hear us come in as she was asleep.
因为他睡着了，所以没听到我回来。
i left a message as you weren’t there.
因为你没在那，所以我留了便条
as monday is a national holiday, all government offices will be close.
周一是一个全国性的节日,所有的政府部门将关闭。
i have to speak english with them, as the guys i live with don’t know any chinese.
我必须说英语,作为球员我生活在不知道任何关于汉语。
for
对前面分句的内容加以解释或说明。
the days were short, for it was now december.
现在是十二月所以天短。
it rained last night, for the ground is wet this morning.
昨晚下雨了,因为今晨地面是潮湿的。
比较：because it rained last night, the ground is wet this morning.
it must be morning, for the birds are singing.
pay attention to your enemies, for they are the first to discover your mistakes.
比较：
as, for引导的主从句，重要性相对平等；
because从句则显得比主句重要，强调原因；
since则从句次于主句，重在说明将要做什么事。