原因狀語(yǔ)從句
　　  
　　比較：because, since, as和for

　　1）because語(yǔ)勢(shì)最強(qiáng)，用來(lái)說(shuō)明人所不知的原因，回答why提出的問(wèn)題。當(dāng)原因是顯而易見(jiàn)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(hào)，則可以用for來(lái)代替。但如果不是說(shuō)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.