This poem is a good sentence in Yan Zhenqing, a great calligrapher and poet in the Tang Dynasty. The whole poem is: "Turn on the light at five o'clock in the middle of the night, just as the man is reading. Black hair doesn't know how to study early, and Bai Shoufang regrets studying late. " "Black hair doesn't know how to study hard early, but a bald head regrets how to study hard late" means: If you don't know how to study hard when you are young, I'm afraid you won't know how to study hard until you are old, and it's too late to regret it.
Zhu's "Persuade to Learn": "Young people are easy to learn, but old people are difficult to become. One inch of time cannot be light. "