Past lives is as simple as a dustpan: simply leave now.
Green mountains and green waters still exist: green mountains are not old, and green waters flow forever.
An old man's epiphyllum will change: but an old friend will wither and grow old like an epiphyllum.
The author's whole poem expresses an emotional artistic conception: time flies, beauty is easy to get old, things are wrong, things are wrong, and they are done and cherished.