A hundred rivers go east to the sea, when will they return to the west? If a person doesn't work hard when he is young, he will regret when he is old ―― laziness when he is young will make him regret when he is old; Reckless youth makes regretful old age; Idle youth, poor old age
The first two sentences: the past time can't go back. The last two sentences: if you don't work hard when you are young, you will regret it when you grow up. Tell people to do what they should do early, don't waste time, and don't wait until they regret it.