英语四级关键英语语法知识：原因状语从句 较为：because, since, as和for
1)because语势强，用于表明人所不知道的缘故，回应why明确提出的难题。当缘故是不言而喻的或无已大家孰知，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正确引导的从句假如放到句末，且前边有逗 号，则可以用for来替代。但要不是表明根本原因，只是多种多样状况多方面推论，就只有用for。 He is absent today, because / for he is ill. He must be ill, for he is absent today.