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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