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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。 | 
