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。
：这几个词按语意的强弱来排，其顺序为：
because—→since—→(as)—→for