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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.

['dʒə:ni] n. 旅行; 行程

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.

Absent ['æbsənt] adj. 缺席的, 不在场的

He must be ill, for he is absent today.