as,because,for 和 since 都可以用来表示原因,其中 because 强度最大,在正式的表示因果关系的句子中,通常不用 as ,因为它容易跟它引导的时间状语从句混淆,让人误解。 一、because 用来说明对听众来说很重要的事情的原因,例如:Because my brother didn’t keep his promise, I couldn’t go fishing.因为我哥哥没有遵守诺言,我不能去钓鱼。Because I woke up late, we missed the bus.因为我起床晚了,我们没赶上公共汽车。当要强调原因时,把 because 放在句末,例如:I will punish you because you lie to me.因为你对我撒谎,我会惩罚你。The students could go home earlier because the teacher was ill.因为老师病了,学生们可以早点回家。二、as 和 since 用来说明听众已经知道的原因,since 更正式,例如:As we lost the final game, we couldn’t qualify for the semi-final.由于我们输掉了最后一场比赛,我们没有资格进入半决赛。Since I was hungry, I wanted to eat something first.因为我饿了,我想先吃点东西。三、for 用来给出新的理由,不能放在句首,强调比较少的直接原因,例如:I had to stay at home and finish my assignment – for the deadline was soon.我不得不呆在家里完成我的任务,因为最后期限很快就到了。We went to a small restaurant - for we were hungry.我们去了一家小餐馆,因为我们饿了。