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。
[注]：这几个词按语意的强弱来排，其顺序为：
because—→since—→(as)—→for
1. because表示直接原因,它所指的原因通常是听话人所不知道的,其语气最强.常用来回答why的提问,一般放于主句之后,也可以单独存在.例如：
(1)I stayed at home because it rained. 因为下雨我呆在家里.
(2)Because Lingling was ill, she didn”t come to school. 玲玲因病,没有上学.
(3)�Why is she absent? 她为什么缺席?
�Because she is sick. 因为她病了.
此外,在强调句型中,只能用because.例如：
(4)It was because I missed the early bus that I was late for school. 我上学迟到是因为我没有赶上早班汽车.
2. since侧重主句,从句表示显然的或已为人所知的理由,常译为“因为”、“既然”,语气比because稍弱,通常置于句首,表示一种含有勉强语气的原因.例如：
(1)Since he asks you, you”ll tell him why. 他既然问你,那就告诉他为什么吧.
(2)Since everyone is here, let”s start. 既然大家都到齐了,我们就出发吧!
(3)Since I understood very little Japanese, I couldn”t follow the conversation. 我日语懂得不多,因而听不懂对话.
3. as是常用词,它表示的“原因”是双方已知的事实或显而易见的原因,或者理由不是很重要,含义与since相同,但语气更弱,没有since正式,常译为“由于,鉴于”.从句说明原因,主句说明结果,主从并重.例如：
(1)We all like her as she is kind. 我们都喜欢她,因为她善良.
(2)As I had a cold, I was absent from school. 因为我感冒了,所以没去上课.
(3)As Xiaowang was not ready, we went without him. 由于小王没有准备好,我们只好不带他去了.
4.
for用作连词时,与because相似,但它所表示的原因往往提供上文未交待过的情况.for不表示直接原因,表明附加或推断的理由,因此for被看作
等立连词,它所引导的分句只能放在句子后部(或单独成为一个句子),并且前后两个分句间的逻辑关系不一定是因果关系,其间用逗号隔开,且for不可置于句
首,for的这一用法常用在书面语中,较正式.例如：
(1)The days are short, for it is now December. 白天短了,因为现在已是十二月份.
(2)It must have rained, for the ground is wet. (从“地面潮湿”作出“下过雨”的推测,但地湿并不一定是下雨所致, for不可以换为because.)
(3)The ground is wet because it has rained. (“下雨”是“地上潮湿”的直接原因.)
前后两个分句间有一定的因果关系时(有时很难区分是直接原因,还是推测性原因),for与because可以互换使用.例如：
(4)I could not go, for / because I was ill. 我没能去,是因为我病了.
(5)He felt no fear, for / because he was a brave boy. 他没有害怕,因为他是个勇敢的男孩本回答被提问者和网友采纳
1. Becasue：当原因作为句子或陈述最重要的一部分时，我们使用because.
Because通常出现在句首部分。
Because is used when the reason is the most important part of the sentence or utterance. The because clause usually comes at the end:
e.g. I went to Spain last summer because I wanted the guarantee of sunshine on every day of my holiday.
2. As和since：当原因是众所周知时，我们使用as和since，因此，这样的原因一般不那么重要。
as或since从句一般置于句首。
As and since are used when the reason is already well known and is therefore usually less important. The as or since clause is usually placed at the beginning of the sentence:
e.g. As the performance had already started, we went up to the balcony and occupied some empty seats there.
e.g. Since John had already eaten, I made do with a sandwich.
3. For：For意味着原因是事后给出的。
For从来都不会置于句首，带有书面英语而非口语的色彩。
For suggests that the reason is given as an afterthought. It is never placed at the beginning of the sentence and is more characteristic of written, rather than spoken English:
e.g. I decided to stop the work I was doing - for it was very late and I wanted to go to bed.本回答被网友采纳
becaue since as和for表达原因时的区分
because,since,as和for的区别
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because和for和since和as当表示原因时的区别是什么啊,
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