Sad sentences and poems about not being able to go back to the past

There is no choice but the flowers fall away, and the familiar swallow returns. Yan Shu's "Huanxisha·A New Song and a Glass of Wine"

The sunset is infinitely beautiful, but it is almost dusk. Li Shangyin's "Le You Yuan/Deng Le You Yuan"

It is easy for a young man to learn from an old man, and it is difficult to learn from an old man, so you should not waste an inch of time. Zhu Xi's "Poem to Encourage Studying/Occasionally"

The prime years will never come again, and it will be difficult to wake up again in one day. Tao Yuanming's "Miscellaneous Poems: Life Without Roots"

A black-haired person does not know how to study diligently early, and a white-haired person regrets studying late.

Yan Zhenqing's "Poem to Encourage Learning"