§49 as/ because/ for/ since Ⅰ. 这三个词都可以用作连词，表“原因、理由” 但有区别： Ⅱ. as “因为、既然”表示的原因或理由是明显的。语意不如because强。当理由是明显的，或者被认为是已知的时，则以用as 为好。如： ① As he was not well, I decided to go without him. 因为他身体不好，我决定独自去了。 ② As it was getting very late, we soon turned back. 因为已很晚了， 我们很快就回来了。 ③ As it is raining, you’d better take a taxi. 既然在下雨，你最好乘出租车。 Ⅲ. because “因为”表直接而明确的原因或理由。即必然的困果关系。在这几个词中，它的语意最强。回答以why 引导的特殊疑问句时，只能用because . 注意：because 不可以与so 连用。如： ① He had to stay at home yesterday because he was ill.昨天他只得呆在家里，因为他病了。 ② I did it because they asked me to do it. 我之所以做这件事，是因为他请我做的。 ③ ---Ｗhy can’t you do it now? --- Because I’m too busy. Ⅳ. since“既然”比as 较为正式，说明为人所知的原因。语气比because 稍弱。 ① Since light travels faster than sound, we see lightening before we hear the thunder.因为光比声音传播得快，所以我们先看到闪电，后听到雷声。 ② Since he can’t answer the question, you’d better ask someone.既然他回答不了这个问题，你最好问别人吧。 ③ Since you are busy, I’ll do it for you.既然你忙，我替你做吧。 Ⅴ. for “因为”表原因或理由时，用以说明理由，只是解释性的。在这几个词中，它的语意最弱。它少用于口语中，也不用于句首。如： ① I asked her to stay to tea, for I had something to tell her. 我请她留下来喝茶，因为我有事要告诉她。 ② We must get rid of carelessness, for it often leads to errors. 我们一定克服粗枝大叶的毛病，因为粗枝大叶常常差错。 ③ It must have rained, for the road is wet. 一定下雨了，因为路是湿的。 ④ I must go now, for my sister is waiting for me. 现在我得走了，我姐姐在等我呢。 ［注］：这几个词按语意的强弱来排，其顺序为： because—→since—→(as)—→for §50 as/ when/ while 这三个词都可以用作连词，表时间关系， 但有区别： Ⅰ. as “当（在）„„时候”往往可与when 或 while 通用，但它着重指主句和从句的动作或事情相并发生。如： ① I saw him as he was getting off the bus. 当下公共汽车的时候，我看见了他。 ② As he walked on, he felt himself getting more and more tired.他继续往前走的时候，感到越来越疲乏。 Ⅱ. when “当（在）„„的时候”主句和从句的动作或事情可以同时发生，也可以先后发生。 ① It was raining when we arrived. 当我们到达的时候，正下着雨。 ② Ｗhen we were at school, we went to the library every day. 我们在求学的时候，每天都到图书馆去。 Ⅲ. while “当（在）„„的时候”表主句的动作或事情发生在从句中的动作或事情的进展过程之中。从句通常为进行时态。它有时可与when通用，但它只能指一段时间（a period of time）,而不能指一点时间（a point of time）如上面的第一个例子。又如： ① Please don’t talk so loud while others are working. 别人在工作的时候，请勿大声讲话。 ② While I am washing the floor, you can be cleaning the windows.我洗地板时，你可以擦窗户。 本文来源：https://www.wddqw.com/doc/8d8e2d1b192e45361066f5e6.html