ecause的语气强于since..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