19.3 原因状语从句
 
比较:because, since, as和for

1) because语势最强,用来说明人所不知的原因,回答why提出的问题。当原因是显而易见的或已为人们所知,就用as或 since。
  I didn't go, because I was afraid.
  Since /As the weather is so bad, we have to delay our journey.

2) 由because引导的从句如果放在句末,且前面有逗 号,则可以用for来代替。但如果不是说明直接原因,而是多种情况加以推断,就只能用for。
  He is absent today, because / for he is ill.
  He must be ill, for he is absent today.






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