19.3 原因狀語從句
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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(dāng)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(dǎo)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.