"向祖父忏悔" can be translated into English as "confess to one's grandfather" or "make a confession to grandfather" .
其中,“confess”意为“忏悔、坦白”,“to one's grandfather”或“to grandfather”表示“向祖父”,这样的表达既保留了原句的核心意思,又符合英语的表达习惯。