“A person with black hair does not know how to study hard early, but a person with white hair regrets studying late.”

This poem is a good line from "Encouraging Learning" written by Yan Zhenqing, a great calligrapher and poet in the Tang Dynasty. . The whole poem is: "The lights are on at three o'clock and the rooster is on the fifth o'clock, which is the time for men to study. Black hair does not know how to study hard early, and white heads regret that it is late to study." "Black hair does not know how to study hard and it is early, and white heads regret that it is late to study" means: If you didn't know how to study hard and study hard when you were young, I'm afraid you won't know how to study hard until you are old and it will be too late and it will be too late to regret.