as
单词辨析：
通常置于句首并用于正式场合，不能像 though 那样用于省略句或引出倒装语序的让步状语从句。
Although it was dark, he still worked in the field.
天黑了，他依然在田里劳作。
We bought the car although it was old.
虽然这辆车是旧的,我们还是买了下来。
作此义解时，只用于倒装语序结构中，语气强于 although 和 though。
Hard as he works, he can't support his family.
他工作努力，可还是无法养活一家人。
Pretty as she is, no one likes her.
她很美丽，但没人喜欢她。
在文体上不如 although 正式，常用于省略句，或引出倒装语序的让步状语从句。
Though (he is) living alone, he is very happy.
他虽然一个人生活，但很愉快。
Though (he is) certainly not a lady-killer, he is still very attractive.
他虽然还不至于让女人一见倾心，却很有魅力。
Strange though it may sound, I was pleased it was over.
尽管听起来也许很奇怪，但我很高兴这都结束了。
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。
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。
如果所述理由是明显的或者已知的，则用 as 较 because 更为合适。
As it's raining, you'd better stay at home.
既然在下雨，你最好呆在家里。
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。
for 属于并列连词，由它引导的分句表示补充说明性的理由，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 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。
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。
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。
所引导的状语从句中，谓语可以是延续性动词，也可以是瞬时动词，其动作既可以和主句的动作同时发生，也可以在主句的动作之前或之后发生。
He got engaged to Lucy when travelling last autumn.
去年秋天旅行时，他与露西订了婚。
It was raining when we arrived.
我们到的时候正在下雨。
Turn off the switch when anything goes wrong with the device.
如果装置出故障，就把开关关掉。
引导状语从句时，侧重指从句中的动作与主句中的动作同时发生或同时进行，一般持续时间不长，其谓语可以是延续性动词，也可以是瞬时动词。
She was coming in as he was going out.
她进来的时候，他正要出去。
I caught a whiff of perfume as she swept past me.
她从我身边走过时，我闻到一股香水味。
Joe jumped onto the bus just as it was pulling away.
正当公共汽车开动时，乔跳上了车。
指在某事发生之际或发生的过程中，另一事又发生。所引导的状语从句中，谓语应为延续性动词，而不用瞬时动词，强调其动作与主句中的动作同时发生，但持续时间一般较长；有时也可指主句的动作在从句的动作过程中发生。
Don't interrupt him while he is thinking.
他在思考时别去打搅他。
She got married while working as a waitress.
她在饭店当服务生期间结婚了。
Pat killed time by watching TV while waiting for her.
帕特在等她时边看电视边打发时间。
相关例句：
The border, effectively closed since 1981, will be opened as of January the 1st
实质上从1981年就已关闭的边境将于1月1日开放。
Enjoy the first hour of the day. This is important as it sets the mood for the rest of the day.
好好享受每天的第一个小时。这之所以重要是因为它会决定你一整天的心情。
Today, as usual, he was wearing a three-piece suit
像往常一样，他今天穿的是三件套。
They should make decisions as to whether the student needs more help
他们应该决定学生是否需要更多的帮助。
The news apparently came as a complete surprise
这个消息显然让人大吃一惊。
The book was banned in the US, as were two subsequent books.
这本书在美国遭禁，随后出版的两本也是一样。