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因为, for引导的是一个并列句,它后面的句子一般作为前面的解释；
because引导的是一个原因状语从句,它强调绝对的,所以why的句子,一定要用because回答.
原因状语从句一般由because, since, as, for引导?注意as,because,since和for的区别:
①如果原因是构成句子的最主要部分,一般用because.because引导的从句一般不放在句子的开头.例如:
I missed the train because I got up late.
注:对于以why开头的问句,一般只能用because引导的从句来回答.
②如果原因已为人们所知,或不如句子的其余部分重要,就用as或since、since比as稍微正式一些.as和since引导的从句一般放在句子的开头.例如:
As he was not well,I decided to go there without him.
Since this method doesn't work, let's try another.
③for表示所说的理由是一种补充说明,因此,for引导的从句可以放在括号里,而且for引导的从句一般不放在句子的开头.例如:
I decided to stop and have lunch, for I was feeling quite hungry.