比較：because, since, as和for
1） because語勢最強，用來說明人所不知的原因，回答why提出的問題。當原因是顯而易見的或已為人們所知，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引導的從句如果放在句末，且前面有逗 號，則可以用for來代替。但如果不是說明直接原因，而是多種情況加以推斷，就只能用for。
he is absent today, because / for he is ill.
he must be ill, for he is absent today.