as/ because/ for/ since

　　Ⅰ. 這三個詞都可以用作連詞，表原因、理由 但有區別：

　　Ⅱ. as 因為、既然表示的原因或理由是明顯的。語意不如because強。當理由是明顯的，或者被認為是已知的時，則以用as 為好。如：

　　① As he was not well, I decided to go without him.

　　因為他身體不好，我決定獨自去了。

　　② As it was getting very late, we soon turned back.

　　因為已很晚了， 我們很快就回來了。

　　③ As it is raining, youd better take a taxi.

　　既然在下雨，你最好乘出租車。

　　Ⅲ. because 因為表直接而明確的原因或理由。即必然的困果關系。在這幾個詞中，它的語意最強。回答以why 引導的特殊疑問句時，只能用because . 注意：because 不可以與so 連用。如：

　　① He had to stay at home yesterday because he was ill.

　　昨天他只得呆在家里，因為他病了。

　　② I did it because they asked me to do it.

　　我之所以做這件事，是因為他請我做的。

　　③ ---Ｗhy cant you do it now? --- Because Im too busy.

　　Ⅳ. since既然比as 較為正式，說明為人所知的原因。語氣比because 稍弱。

　　① Since light travels faster than sound, we see lightening before we hear the thunder.

　　因為光比聲音傳播得快，所以我們先看到閃電，后聽到雷聲。

　　② Since he cant answer the question, youd better ask someone.

　　既然他回答不了這個問題，你最好問別人吧。

　　③ Since you are busy, Ill do it for you.既然你忙，我替你做吧。

　　Ⅴ. for 因為表原因或理由時，用以說明理由，只是解釋性的。在這幾個詞中，它的語意最弱。它少用于口語中，也不用于句首。如：

　　① I asked her to stay to tea, for I had something to tell her.

　　我請她留下來喝茶，因為我有事要告訴她。

　　② We must get rid of carelessness, for it often leads to errors.

　　我們一定克服粗枝大葉的毛病，因為粗枝大葉常常差錯。

　　③ It must have rained, for the road is wet.

　　一定下雨了，因為路是濕的。

　　④ I must go now, for my sister is waiting for me.

　　現在我得走了，我姐姐在等我呢。

　　［注］：這幾個詞按語意的強弱來排，其順序為：

　　becausesince(as)for

　　as/ because/ for/ since

　　Ⅰ. 這三個詞都可以用作連詞，表原因、理由 但有區別：

　　Ⅱ. as 因為、既然表示的原因或理由是明顯的。語意不如because強。當理由是明顯的，或者被認為是已知的時，則以用as 為好。如：

　　① As he was not well, I decided to go without him.

　　因為他身體不好，我決定獨自去了。

　　② As it was getting very late, we soon turned back.

　　因為已很晚了， 我們很快就回來了。

　　③ As it is raining, youd better take a taxi.

　　既然在下雨，你最好乘出租車。

　　Ⅲ. because 因為表直接而明確的原因或理由。即必然的困果關系。在這幾個詞中，它的語意最強。回答以why 引導的特殊疑問句時，只能用because . 注意：because 不可以與so 連用。如：

　　① He had to stay at home yesterday because he was ill.

　　昨天他只得呆在家里，因為他病了。

　　② I did it because they asked me to do it.

　　我之所以做這件事，是因為他請我做的。

　　③ ---Ｗhy cant you do it now? --- Because Im too busy.

　　Ⅳ. since既然比as 較為正式，說明為人所知的原因。語氣比because 稍弱。

　　① Since light travels faster than sound, we see lightening before we hear the thunder.

　　因為光比聲音傳播得快，所以我們先看到閃電，后聽到雷聲。

　　② Since he cant answer the question, youd better ask someone.

　　既然他回答不了這個問題，你最好問別人吧。

　　③ Since you are busy, Ill do it for you.既然你忙，我替你做吧。

　　Ⅴ. for 因為表原因或理由時，用以說明理由，只是解釋性的。在這幾個詞中，它的語意最弱。它少用于口語中，也不用于句首。如：

　　① I asked her to stay to tea, for I had something to tell her.

　　我請她留下來喝茶，因為我有事要告訴她。

　　② We must get rid of carelessness, for it often leads to errors.

　　我們一定克服粗枝大葉的毛病，因為粗枝大葉常常差錯。

　　③ It must have rained, for the road is wet.

　　一定下雨了，因為路是濕的。

　　④ I must go now, for my sister is waiting for me.

　　現在我得走了，我姐姐在等我呢。

　　［注］：這幾個詞按語意的強弱來排，其順序為：

　　becausesince(as)for