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.