原因狀語從句
比較：because， since， as和for
1)because語勢，用來說明人所不知的原因，回答why提出的問題。當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.
比較：because， since， as和for
1)because語勢，用來說明人所不知的原因，回答why提出的問題。當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.