你能告诉我今天迟到的原因吗？

Can you tell me the reason why you are late today?
呵呵！我用了个定语从句
我觉得这样更能体现出你的英语功底
写作文也是这样哦
要加点从句，这样老师会多给你分的！
说明一下
以reason为先行词的定语从句不是一定跟why的
楼主要注意哦

Could you please tell me the reason why you are late today?

Can you tell me the reason for being late today?