Ⅰ. 這三個(gè)詞都可以用作連詞，表“原因、理由” 但有區(qū)別：
Ⅱ. as “因?yàn)椤⒓热弧北硎镜脑蚧蚶碛墒敲黠@的。語(yǔ)意不如because強(qiáng)。當(dāng)理由是明顯的，或者被認(rèn)為是已知的時(shí)，則以用as 為好。如：
① As he was not well, I decided to go without him. 因?yàn)樗眢w不好，我決定獨(dú)自去了。
② As it was getting very late, we soon turned back. 因?yàn)橐押芡砹耍?我們很快就回來(lái)了。
③ As it is raining, you’d better take a taxi. 既然在下雨，你乘出租車。
Ⅲ. because “因?yàn)椤北碇苯佣鞔_的原因或理由。即必然的困果關(guān)系。在這幾個(gè)詞中，它的語(yǔ)意?；卮鹨詗hy 引導(dǎo)的特殊疑問(wèn)句時(shí)，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?yàn)樗×恕?BR> ② I did it because they asked me to do it. 我之所以做這件事，是因?yàn)樗?qǐng)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(shuō)明為人所知的原因。語(yǔ)氣比because 稍弱。
① Since light travels faster than sound, we see lightening before we hear the thunder.因?yàn)楣獗嚷曇魝鞑サ每?，所以我們先看到閃電，后聽(tīng)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(gè)問(wèn)題，你問(wèn)別人吧。
③ Since you are busy, I’ll do it for you.既然你忙，我替你做吧。
Ⅴ. for “因?yàn)椤北碓蚧蚶碛蓵r(shí)，用以說(shuō)明理由，只是解釋性的。在這幾個(gè)詞中，它的語(yǔ)意最弱。它少用于口語(yǔ)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(qǐng)她留下來(lái)喝茶，因?yàn)槲矣惺乱嬖V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?yàn)榇种Υ笕~常常差錯(cuò)。
③ It must have rained, for the road is wet. 一定下雨了，因?yàn)槁肥菨竦摹?BR> ④ I must go now, for my sister is waiting for me. 現(xiàn)在我得走了，我姐姐在等我呢。
[注]：這幾個(gè)詞按語(yǔ)意的強(qiáng)弱來(lái)排，其順序?yàn)椋?BR> because—→since—→(as)—→for
Ⅱ. as “因?yàn)椤⒓热弧北硎镜脑蚧蚶碛墒敲黠@的。語(yǔ)意不如because強(qiáng)。當(dāng)理由是明顯的，或者被認(rèn)為是已知的時(shí)，則以用as 為好。如：
① As he was not well, I decided to go without him. 因?yàn)樗眢w不好，我決定獨(dú)自去了。
② As it was getting very late, we soon turned back. 因?yàn)橐押芡砹耍?我們很快就回來(lái)了。
③ As it is raining, you’d better take a taxi. 既然在下雨，你乘出租車。
Ⅲ. because “因?yàn)椤北碇苯佣鞔_的原因或理由。即必然的困果關(guān)系。在這幾個(gè)詞中，它的語(yǔ)意?；卮鹨詗hy 引導(dǎo)的特殊疑問(wèn)句時(shí)，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?yàn)樗×恕?BR> ② I did it because they asked me to do it. 我之所以做這件事，是因?yàn)樗?qǐng)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(shuō)明為人所知的原因。語(yǔ)氣比because 稍弱。
① Since light travels faster than sound, we see lightening before we hear the thunder.因?yàn)楣獗嚷曇魝鞑サ每?，所以我們先看到閃電，后聽(tīng)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(gè)問(wèn)題，你問(wèn)別人吧。
③ Since you are busy, I’ll do it for you.既然你忙，我替你做吧。
Ⅴ. for “因?yàn)椤北碓蚧蚶碛蓵r(shí)，用以說(shuō)明理由，只是解釋性的。在這幾個(gè)詞中，它的語(yǔ)意最弱。它少用于口語(yǔ)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(qǐng)她留下來(lái)喝茶，因?yàn)槲矣惺乱嬖V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?yàn)榇种Υ笕~常常差錯(cuò)。
③ It must have rained, for the road is wet. 一定下雨了，因?yàn)槁肥菨竦摹?BR> ④ I must go now, for my sister is waiting for me. 現(xiàn)在我得走了，我姐姐在等我呢。
[注]：這幾個(gè)詞按語(yǔ)意的強(qiáng)弱來(lái)排，其順序?yàn)椋?BR> because—→since—→(as)—→for