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, you’d better take a taxi. 既然在下雨，妳最好乘出租車。
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 . 註意：because 不可以與so 連用。如：
① He had to stay at home yesterday because he was ill.昨天他只得呆在家裏，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 can’t you do it now? --- Because I’m 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，後聽到雷聲。
② Since he can’t answer the question, you’d better ask someone.既然他回答不了這個問題，妳最好問別人吧。
③ Since you are busy, I’ll do it for you.既然妳忙，我替妳做吧。
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。它少用於口語中，也不用於句首。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. 我們壹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. 壹定下雨了，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。
[註]：這幾個詞按語意的強弱來排，其順序為：
because—→since—→(as)—→for