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, youd 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 cant you do it now? --- Because Im 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 cant answer the question, youd 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, Ill 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：

　　becausesince(as)for

　　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, youd 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 cant you do it now? --- Because Im 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 cant answer the question, youd 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, Ill 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：

　　becausesince(as)for