because,since,as,for表示原因时有何区别?
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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.

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