because, since, as和for 引导的原因状语从句

  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.