for +句子,for是并列连词,表原因,语气弱,所以只能放在后面（并列分句+for+并列分句）
He can't come to school for he is ill in bed now.
for 为什么不能放在句首?顺便讲一下和它类似的词
for +句子,for是并列连词,表原因,语气弱,所以只能放在后面（并列分句+for+并列分句）
He can't come to school for he is ill in bed now.