逗号可以连接两个意义并列的句子,如你的第一句；也可连接主从句,如你的地二句,但一定是从句在前时用逗号,如主句在前则不加逗号,如：
She refused to reply to his letters although she wanted to see him again.
第一句是倒桩句,强调：many a time.
Many a time have the children been told not to play with fire,but they just turn a deaf ear to the warnings.===The children have been told not to play with fire many a time,but they just turn a deaf ear to the warnings.
这里不能改成你所说的句子.